Hey Joomlers! Here is a book that you might be interested in. Programming Joomla Plugins written by Jisse Reitsma.
Jisse Reitsma is a PHP developer from the Netherlands and founder of Yireo, an internet company providing training but also offering various extensions (free and commercial) for Joomla and Magento.
His book focuses on one thing: Joomla plugins.
Once you have coded a bit with Joomla, you will find plugins to be the most flexible and adaptable part. With plugins, you can modify any Joomla form, any page, any user creation and so on. In his book, Jisse describes the basics of how to get started with plugins and how to write your first Content Plugin. However, he does not stop at the basics:
He also discusses various in-depth topics like login hashes, Sphinx integration, JFinder, design patterns, PHPUnit and Phing.
If you have never bought a Joomla programming book, then this should be your first!
Get your copy: https://www.yireo.com/books/programming-joomla-plugins-book