I wrote Professional XMPP Programming with JavaScript and jQuery (http://professionalxmpp.com), which is a book about developing XMPP applications (and not just chat related) using the Strophe.js (http://strophe.im/strophejs) library.

There are a few books about the XMPP protocol itself, but I really wanted to see something that was more about how to get things done with it than schema descriptions. The book has a multiplayer game, collaborative whiteboard, shared document editor, and the usual chat-inspired examples.

People seem to enjoy it, and I had a lot of fun writing it.