WordPress 2.7 Complete by April Hodge Silver and Hasin Hayder is a great resource for bloggers and web designers to get a good handle on how the WordPress system works, where its strengths and weaknesses lie, and how to customize it to suit your needs.
The book covers both interface training and code development with equal facility. If you merely want to learn how to install the application, add themes and plug-ins, and get blogging, there is plenty of information here for you. If you want to dive into PHP to create your own themes, plug-ins, widgets, and so on, the book offers step-by-step instructions for this as well. It also covers how you can use WordPress as a complete CMS, which is helpful since WordPress has long had the reputation of being merely a blogging tool.