I have always been a big fan of the O'Reilly books, and the Missing Manual series is no exception. I recently had the opportunity to read Wikipedia: The Missing Manual by 
John Broughton.
If you have ever thought about contributing to Wikipedia, or have done so in the past, this is the definitive book on how to contribute and become a successful Wikipedia editor.
The book is broken down into 5 major sections which cover:
- Editing, Creating, and Maintaining Articles
- Collaborating with Other Editors
- Formatting and Illustrating Articles
- Building a Stronger Encyclopedia
- Customizing Wikipedia
Each section is very well written and presents the material in a well organized and easy to understand manner. There is information for everyone from the novice to the experienced editor.
This book is not intended for the casual Wikipedia user, although reading it will certainly provide a lot more insight in to the scale of the entire Wikipedia project. It assumes a certain base level of understanding about what Wikipedia is and why it has become one of the world's largest group writing project. Instead it provides a very complete and concise set of guidance on how to contribute and get it right from the start.