Wikipedia: The Missing Manual is a how-to guide that explains the process of contributing to the English Wikipedia, both for novice users and experienced editors. It was originally written in 2008 by John Broughton, but has since been expanded and updated by many other Wikipedia contributors.
Wikipedia has a myriad of pages documenting policies, guidelines and processes. It's hard to know where to start to learn about contributing to Wikipedia. That's what this book is for: It provides a clear learning path for the essentials, and tells you where to read more if you need it.