These are the parts of an amazing series by Brett McLaughlin at IBM. This is not a easy/fast read so suggested putting away some time(hour or two) before getting your hands into it.

Gain a complete understanding of HTTP status codes, ready states, and the XMLHttpRequest object

For many Web developers, making simple requests and receiving simple responses is all they’ll ever need, but for developers who want to master Ajax, a complete understanding of HTTP status codes, ready states, and the XMLHttpRequest object is required. In this article, Brett McLaughlin will show you the different status codes and demonstrate how browsers handle each and he will showcase the lesser-used HTTP requests that you can make with Ajax.

Mastering Ajax, Part 1

Mastering Ajax, Part 2

Mastering Ajax, Part 3