Joomla 1.5 Development Cookbook is a recent book about Joomla extension development, published by Packt Publishing.
The book is written by James Kennard, an extensions developer with more than 6 years of experience with Joomla.
When browsing Amazon.com today, I noticed a new book coming in November, available right now for pre-order:
Written by bestselling Beginning Joomla! author Dan Rahmel, Advanced Joomla! provides you with the technical know-how and bonanza of information that allows you to take your Joomla! sites to the next level.
About the Book 'Advanced Joomla!'
Advanced Joomla! allows you to study beginning and advanced Joomla! topics back-to-back without requiring any additional outside knowledge. Moreover, it offers you advanced techniques for customizing a Joomla! CMS, including creating templates, administration, and building extensions. Integrate advanced Web 2.0 features into you Joomla! site, including social networking, blogging, and Google and Yahoo web services. Construct advanced Joomla! templates that use multiple stylesheets; third-party frameworks including MooTools and Yahoo! User Interface (YUI) Library for interactive display; specialty definitions for mobile users; and more!