Ajax Patterns and Best Practices by Christian Gross
If you’ve read the previously mentioned books or any tutorials on Ajax, you know enough to get started and put together some simple applications or sites. Ajax Patterns and Best Practices will help you take your knowledge to that next level where you can begin working on more complicated sites that might need to eventually scale. It will also ensure that your code stays beautiful and easy to understand.
This is a book that you should only consider buying if you see yourself working with a lot of complicated Ajax functionality. Otherwise, you really won’t need it.