Scott Raymond latest book, Ajax on Rails, has recently been published by O’Reilly. He also wrote What’s New in Prototype 1.5? on XML.com regarding the latest changes for Prototype 1.5.

• The book demonstrates how to use Ajax and Rails to build
  high-performance, scalable applications without having to write reams of
  difficult-to-maintain JavaScript code.
• Provides a thorough, low-level understanding of what’s under the hood,
  with a high-level understanding of how to use Ajax techniques
  effectively.
• Includes a complete reference to Prototype 1.5 and script.aculo.us 1.6.1,
  which can be used even outside of the Rails platform.
• Three fully worked out applications—a quiz program, a photo gallery,
  and a workgroup collaboration tool—show how everything fits together
  in practice.

— taken from the book’s Full Description