This book will help you through the most common tasks you may encounter when developing a WordPress theme. However, you'll certainly have questions along the way that need further investigation. When these questions arise, you should consult the official WordPress documentation.
First fire up your browser and go to http://codex.wordpress.org. This is the home page for the codex, where you'll spend a lot of time as a WordPress developer. The page looks like the following screenshot:
From the home page, you can browse to a topic that you are interested in. In the following screenshot, you can see the topic page for the WordPress Database Description:
The WordPress Codex is a user-generated set of documentation for the WordPress system. Most functions and functionality are well-documented, and most theme-related questions can be answered through careful browsing and reading.
For questions related to specific topics, it may be useful to consult that topic's page. Here is a short guide:
Theme development—http://codex.wordpress.org/Theme_Development
Template tags—http://codex.wordpress.org/Template_Tags
Conditional tags—http://codex.wordpress.org/Conditional_Tags
Function reference—http://codex.wordpress.org/Function_Reference