Books:

Ajax is a subset of JavaScript; it stands for "Asynchronous JavaScript and XML". Ajax enables your web pages to send and receive messages from the web server without requiring users to reload the page.

Ajax Basics

• Mastering Ajax, Part 1: Introduction to Ajax. A developerWorks document from IBM.
• Mastering Ajax, Part 2: Make asynchronous requests with JavaScript and Ajax, from IBM. Learn how to write the code necessary for Ajax calls.
• Dynamic HTML and XML: The XMLHttpRequest Object - an excellent intro and tutorial on the Apple website.
• XML.com: Very Dynamic Web Interfaces covers the XMLHttpRequest and also includes an example of a (PHP) server-side script to handle the Ajax request.

Tutorials and Useful Stuff

• Round-up of 30 AJAX Tutorials, by Max Keisler.
• 80+ Ajax Solutions for Professional Coding, collected by Smashing Magazine.

Resources

• Ajaxian offers news and tips for creating cool Ajax apps.