I just stumbled across a book called Ajax in Action and it is wonderful. I had no idea this book was even out. It provides a more comprehensive overview of Ajax technologies than any book on the market (or even in beta) including a great section on Ajax Design Patterns.
The coverage is nice and varied. They show you how to write Ajax from scratch first, including lots of details about how it works and how to use it well in your application. They then go on to detail the many libraries and frameworks available, and show you how to refactor your code with these libraries. Finally, there are plenty of examples of Ajax applications toward the end of the book.
Highly recommended for anyone developing what you might call a Web 2.0 application!