Read this book, and you'll:
Get an overview of the basic technologies you'll need to use to build Ajax applications.
Understand the XMLHttpRequest object, the core of Ajax applications.
Build a cross-browser XMLHttpRequest wrapper that you can use in all of your Ajax projects.
Build an Ajax application monitor that works in real time.
Learn how to use Ajax without breaking older browsers or causing accessibility issues.
Get your Ajax application working with screen readers.
Use dynamic animations to improve usability.
Build edit-in-place functionality.
Create an Ajax search application that pulls data from Amazon, del.icio.us, and Google.
Fix the "Back button" problem.
Use JSON, YAML, and XML to communicate with the server.
Build an Ajax drag 'n' drop chess game.
And a whole lot more!
Throughout the text, the author stresses usability, accessibility, and graceful degradation for older, less-capable web browsers.