Modern-day software development is often facilitated by the reuse of third-party software libraries. Despite the significant effort to understand the factors contributing to library selection, it is relatively unknown how the libraries are selected and what tools are still needed to support the selection process. Using Straussian grounded theory, we conducted and analyzed the interviews of 24 professionals across the world and derived a model of library selection process which is governed by six selection patterns (i.e., rules). The model draws from marketing theory and lays the groundwork for the development of a library selection tool which captures the technical and non-technical aspects developers consider.