Mastering Joomla! 1.5 Extension and Framework Development Second Edition by James Kennard

Packed with expert advice, working examples, and tips, this book is an essential reference guide that you will turn to again and again as you work with Joomla!. PHP developers who want to understand how Joomla! Works, with a view to advanced customization or extension development, and who want to learn how to work with the Joomla! Framework.