The Second Edition of Joe LeBlanc's book "Learning Joomla! 1.5 Extension Development" is now available from PackT. Normally, I would recommend buying the book through the Joomla! Shop since the project gets an added bonus, but the title is not yet available. (When it is available, I will update this post.)
If you want to develop using Joomla!, or if you are looking to strengthen your skills, I recommend this step-by-step guide to creating Joomla! 1.5 Components, Modules, and Plugins. By following the examples and instruction in the book, you can learn to build a fully functioning set of extensions, complete with SEF URLs, the ability for visitor interaction (including comments) and backend administrative capabilities. Downloadable code examples are provided with the book.
"Learning Joomla! 1.5 Extension Development" covers all of the important development topics: email, file uploads, language files and translations, debugging, and packaging. If you want to develop using Joomla! 1.5, this book will show you the way.
I also recommend James Kennard's "Mastering Joomla! 1.5 Extension and Framework Development." Where LeBlanc's book might be described as a "Quick Start" for developers, Kennard's is more of a reference, documenting the detail of the Joomla! API and explaining how to take advantage of the rich application development framework. While I am developing, I frequently have Kennard's book open.
The pair are a perfect combination to help a developer get an excellent start and provide the necessary support as skills grow.
If you want to learn to build Joomla! extensions and someone you know is asking "What do you want for the holidays?" (and this is out of the question), consider these titles. It's always helpful to purchase your Joomla! books through the Joomla! Shop. In doing so, you help make a portion of the purchase available to the project, which is another excellent gift idea for the holidays. Right now, however, Amazon does not have the Second Edition available, so you will need to purchase the book directly from PackT.