public class DataGridRightClickBehavior : Behavior<DataGrid>
{
    public ICommand SaveSelectedCommand
    {
        get { return (ICommand)GetValue(SaveSelectedCommandProperty); }
        set { SetValue(SaveSelectedCommandProperty, value); }
    }

    // Using a DependencyProperty as the backing store for SaveSelectedCommandProperty.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty SaveSelectedCommandProperty =
        DependencyProperty.Register("SaveSelectedCommand", typeof(ICommand),
            typeof(DataGridRightClickBehavior), new PropertyMetadata(null));

    private ContextMenu contextMenu;

    protected override void OnAttached()
    {
        base.OnAttached();
        contextMenu = new ContextMenu();
        var menuItem = new MenuItem
        {
            Header = "Save Selected Items"
        };
        menuItem.Click += MenuItem_Click;
        contextMenu.Items.Add(menuItem);

        AssociatedObject.ContextMenu = contextMenu;
    }

    private void MenuItem_Click(object sender, RoutedEventArgs e)
    {
        SaveSelectedCommand?.Execute(null);
    }

    protected override void OnDetaching()
    {
        base.OnDetaching();

        if (contextMenu != null)
        {
            contextMenu.Items.Clear();
            contextMenu = null;
        }
    }
}

Install-Package Microsoft.Xaml.Behaviors.Wpf
Install-Package Newtonsoft.Json

<Window x:Class="WpfApp11.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:behavior="http://schemas.microsoft.com/xaml/behaviors"
        xmlns:local="clr-namespace:WpfApp11"
        mc:Ignorable="d"
        WindowState="Maximized"
        Title="{Binding MainTitle}">
    <Window.DataContext>
        <local:MainVM/>
    </Window.DataContext>
    <Window.Resources>
        <local:ImgConverter x:Key="ImgConverter"/>
    </Window.Resources>
    <Grid>
        <DataGrid ItemsSource="{Binding BooksCollection}"
                  SelectionMode="Extended"
                  IsReadOnly="True"
                  CanUserAddRows="True"
                  AutoGenerateColumns="False"
                  VirtualizingPanel.IsVirtualizing="True"
                  VirtualizingPanel.VirtualizationMode="Recycling"
                  VirtualizingPanel.CacheLength="3,3"
                  VirtualizingPanel.CacheLengthUnit="Item"
                  ScrollViewer.CanContentScroll="True"
                  ScrollViewer.IsDeferredScrollingEnabled="True"
                  EnableRowVirtualization="True"
                  EnableColumnVirtualization="True"
                  UseLayoutRounding="True"
                  SnapsToDevicePixels="True">
            <behavior:Interaction.Triggers>
                <behavior:EventTrigger EventName="SelectionChanged">
                    <behavior:InvokeCommandAction Command="{Binding SelectedBooksCommand}"
                                                  CommandParameter="{Binding SelectedItems,RelativeSource={RelativeSource Mode=FindAncestor,AncestorType={x:Type DataGrid}}}">
                    </behavior:InvokeCommandAction>
                </behavior:EventTrigger>
            </behavior:Interaction.Triggers>

            <behavior:Interaction.Behaviors>
                <local:DataGridRightClickBehavior SaveSelectedCommand="{Binding SaveSelectedItemsCommand}"/>
            </behavior:Interaction.Behaviors>
            <DataGrid.Columns>
                <DataGridTemplateColumn>
                    <DataGridTemplateColumn.CellTemplate>
                        <DataTemplate>
                            <Grid Width="{x:Static SystemParameters.FullPrimaryScreenWidth}" 
                                  Height="{x:Static SystemParameters.FullPrimaryScreenHeight}">
                                <Grid.Resources>
                                    <Style TargetType="TextBlock">
                                        <Setter Property="FontSize" Value="30"/>
                                        <Style.Triggers>
                                            <Trigger Property="IsMouseOver" Value="True">
                                                <Setter Property="FontSize" Value="50"/>
                                                <Setter Property="Foreground" Value="Red"/>
                                            </Trigger>
                                        </Style.Triggers>
                                    </Style>
                                </Grid.Resources>
                                <Grid.Background>
                                    <ImageBrush ImageSource="{Binding ImgUrl,Converter={StaticResource ImgConverter}}"
                     Stretch="Uniform"/>
                                </Grid.Background>
                                <Grid.RowDefinitions>
                                    <RowDefinition/>
                                    <RowDefinition/>
                                </Grid.RowDefinitions>
                                <Grid.ColumnDefinitions>
                                    <ColumnDefinition/>
                                    <ColumnDefinition/>
                                </Grid.ColumnDefinitions>
                                <TextBlock Text="{Binding Id}" Grid.Row="0" Grid.Column="0"/>
                                <TextBlock Text="{Binding Name}" Grid.Row="0" Grid.Column="1"/>
                                <TextBlock Text="{Binding ISBN}" Grid.Row="1" Grid.Column="0" Grid.ColumnSpan="2"/>
                            </Grid>
                        </DataTemplate>
                    </DataGridTemplateColumn.CellTemplate>
                </DataGridTemplateColumn>
            </DataGrid.Columns>
        </DataGrid>
    </Grid>
</Window>
using Microsoft.Xaml.Behaviors;
using Newtonsoft.Json;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Diagnostics;
using System.Globalization;
using System.IO;
using System.Runtime.CompilerServices;
using System.Runtime.Intrinsics.X86;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;
using System.Xml;
using Formatting = Newtonsoft.Json.Formatting;

namespace WpfApp11
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            this.Loaded += async (s, e) =>
            {
                var vm = this.DataContext as MainVM;
                if (vm != null)
                {
                    await vm.InitBooksCollection(5345678);
                }
            };
        }
    }

    public class MainVM : INotifyPropertyChanged
    {
        public MainVM()
        {
            //Task.Run(async () =>
            //{
            //    await InitBooksCollection(15000008);
            //});
            InitCommands();
        }

        private void InitCommands()
        {
            SelectedBooksCommand = new DelCommand(SelectedBooksCommandExecuted);
            SaveSelectedItemsCommand = new DelCommand(SaveSelectedItemsCommandExecuted);
        }

        private void SaveSelectedItemsCommandExecuted(object? obj)
        {
            Task.Run(() =>
            {
                if (selectedBooksList.Any())
                {
                    var jsonStr = JsonConvert.SerializeObject(selectedBooksList, Formatting.Indented);
                    var jsonFile = $"Json_{DateTime.Now.ToString("yyyyMMddHHmmssffff")}.json";
                    using (StreamWriter jsonWriter = new StreamWriter(jsonFile, false, Encoding.UTF8))
                    {
                        jsonWriter.WriteLine(jsonStr);
                        System.Diagnostics.Debug.WriteLine($"{DateTime.Now}\nSaved {selectedBooksList.Count} items to {jsonFile}");
                    }
                }
            });
        }

        private void SelectedBooksCommandExecuted(object? obj)
        {
            Task.Run(() =>
            {
                selectedBooksList = ((System.Collections.IList)obj).Cast<Book>()?.ToList();
                System.Diagnostics.Debug.WriteLine($"{DateTime.Now},Selected {selectedBooksList.Count} items");
            });

        }

        private List<Book> selectedBooksList = new List<Book>();

        public ICommand SelectedBooksCommand { get; set; }
        public ICommand SaveSelectedItemsCommand { get; set; }

        public async Task InitBooksCollection(int cnt)
        {
            var imgDir = @"../../../Images";
            if (!Directory.Exists(imgDir))
            {
                return;
            }
            var imgs = Directory.GetFiles(imgDir);
            if (imgs == null || !imgs.Any())
            {
                return;
            }

            int imgsCount = imgs.Count();
            BooksCollection = new ObservableCollection<Book>();
            List<Book> bksList = new List<Book>();
            for (int i = 1; i < cnt + 1; i++)
            {
                bksList.Add(new Book()
                {
                    Id = i,
                    ImgUrl = imgs[i % imgsCount],
                    ISBN = $"ISBN_{i}_{Guid.NewGuid():N}",
                    Name = $"Name_{i}"
                });

                if (i % 500000 == 0)
                {
                    await PopulateBooksCollectionAsync(bksList);
                }
            }

            if (bksList.Any())
            {
                await PopulateBooksCollectionAsync(bksList);
            }
        }

        private async Task PopulateBooksCollectionAsync(List<Book> bksList)
        {
            var tempList = bksList.ToList();
            await Application.Current.Dispatcher.InvokeAsync(() =>
            {
                foreach (var bk in tempList)
                {
                    BooksCollection.Add(bk);
                }
                MainTitle = $"{DateTime.Now.ToString("O")},loaded {BooksCollection.Count} items,memory:{GetMem()}";
                bksList.Clear();
            }, System.Windows.Threading.DispatcherPriority.Background);
        }

        private string GetMem()
        {
            return $"{Process.GetCurrentProcess().PrivateMemorySize64 / 1024 / 1024:N2} M";
        }

        private string mainTitle="";
        public string MainTitle
        {
            get
            {
                return mainTitle;
            }
            set
            {
                if (value != mainTitle)
                {
                    mainTitle = value;
                    OnPropertyChanged();
                }
            }
        }

        private ObservableCollection<Book> booksCollection;
        public ObservableCollection<Book> BooksCollection
        {
            get
            {
                return booksCollection;
            }
            set
            {
                if (value != booksCollection)
                {
                    booksCollection = value;
                    OnPropertyChanged();
                }
            }
        }

        public event PropertyChangedEventHandler? PropertyChanged;
        private void OnPropertyChanged([CallerMemberName] string propertyName = "")
        {
            var handler = PropertyChanged;
            handler?.Invoke(this, new PropertyChangedEventArgs(propertyName));
        }
    }

    public class ImgConverter : IValueConverter
    {
        private readonly Dictionary<string, ImageSource> imgCache = new Dictionary<string, ImageSource>();
        private object lockObj = new object();
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            try
            {
                var imgUrl = value as string;
                if (string.IsNullOrEmpty(imgUrl) || !File.Exists(imgUrl))
                {
                    return null;
                }

                lock (lockObj)
                {
                    if (imgCache.TryGetValue(imgUrl, out ImageSource img))
                    {
                        return img;
                    }
                }

                using (var fs = new FileStream(imgUrl, FileMode.Open, FileAccess.Read, FileShare.Read))
                {
                    var bmp = new BitmapImage();
                    bmp.BeginInit();
                    bmp.StreamSource = fs;
                    bmp.CacheOption = BitmapCacheOption.OnLoad;
                    bmp.EndInit();

                    if(bmp.CanFreeze)
                    {
                        bmp.Freeze();
                    }

                    lock(lockObj)
                    {
                        if(!imgCache.ContainsKey(imgUrl))
                        {
                            imgCache.Add(imgUrl, bmp);
                        }
                    }
                    return bmp;
                }

            }
            catch (Exception ex)
            {
                System.Diagnostics.Debug.WriteLine($"{ex.Message}", $"{DateTime.Now.ToString("O")}");
            }
            return null;
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            return DependencyProperty.UnsetValue;
        }
    }

    public class DelCommand : ICommand
    {
        private Action<object?>? execute;
        private Predicate<object?>? canExecute;
        public DelCommand(Action<object?>? executeValue, Predicate<object?>? canExecuteValue = null)
        {
            execute = executeValue;
            canExecute = canExecuteValue;
        }

        public event EventHandler? CanExecuteChanged
        {
            add
            {
                CommandManager.RequerySuggested += value;
            }
            remove
            {
                CommandManager.RequerySuggested -= value;
            }
        }

        public bool CanExecute(object? parameter)
        {
            return canExecute == null ? true : canExecute(parameter);
        }

        public void Execute(object? parameter)
        {
            execute?.Invoke(parameter);
        }
    }

    public class DataGridRightClickBehavior : Behavior<DataGrid>
    {
        public ICommand SaveSelectedCommand
        {
            get { return (ICommand)GetValue(SaveSelectedCommandProperty); }
            set { SetValue(SaveSelectedCommandProperty, value); }
        }

        // Using a DependencyProperty as the backing store for SaveSelectedCommandProperty.  This enables animation, styling, binding, etc...
        public static readonly DependencyProperty SaveSelectedCommandProperty =
            DependencyProperty.Register("SaveSelectedCommand", typeof(ICommand),
                typeof(DataGridRightClickBehavior), new PropertyMetadata(null));

        private ContextMenu contextMenu;

        protected override void OnAttached()
        {
            base.OnAttached();
            contextMenu = new ContextMenu();
            var menuItem = new MenuItem
            {
                Header = "Save Selected Items"
            };
            menuItem.Click += MenuItem_Click;
            contextMenu.Items.Add(menuItem);

            AssociatedObject.ContextMenu = contextMenu;
        }

        private void MenuItem_Click(object sender, RoutedEventArgs e)
        {
            SaveSelectedCommand?.Execute(null);
        }

        protected override void OnDetaching()
        {
            base.OnDetaching();

            if (contextMenu != null)
            {
                contextMenu.Items.Clear();
                contextMenu = null;
            }
        }
    }

    public class Book
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public string ISBN { get; set; }
        public string ImgUrl { get; set; }
    }
}