Although there’s nowhere better to sharpen your skills than your favorite search engine results, I’m still a fan of a good book and thought I would mention a few that I like and use regularly.
Professional WordPress is, I think, the best book on wordpress to date. Released in 2010 from Wrox, this book covers a lot of ground, starting with the basics and moving through to more advanced topics. Highlights include a great section on building plugins and a thorough discussion of the different methods of building custom loops and when to use them.
Jquery: Novice to Ninja
Front End Drupal
With the increasing popularity of WordPress as a full featured c.m.s., I spend most of my time working with it, rather than some of the other options, such as Drupal. But Drupal is a powerful c.m.s. and excels in a lot of areas that WordPress just isn’t built for, particularly straight out of the box. Front End Drupal covers all aspects of building custom themes for Drupal.
PHP 6 and MySql 5
PHP 6 and MySql 5 by author Larry Ullman offers a solid introduction to the php and mysql languages and how they can be used to build web applications. And when you’re done with this one give php 5 Advanced a try. This one covers advanced php networking, object oriented programming and a lot more.