Wikipedia: The Missing Manual is popular how-to book on Wikipedia that has all the information you need to get started with editing pages on Wikipedia.
"Wikipedia: The Missing Manual gives you practical advice on creating articles and collaborating with fellow editors, improving existing articles, and working with the Wikipedia community to review new articles, mediate disputes, and maintain the site."
The book sells for $30 on Amazon but now the publishers have uploaded the entire book to Wikipedia website under the GNU license. You can not only read the Wikipedia book for free but the license even lets you copy, translate and distribute the book in any format provided the derivative work is also free.
And like all other pages on Wikipedia, the new "Wikipedia: The Missing Manual" section too is open for public editing. Thanks Sara Peyton.
Also see: How to Use Wikipedia More Effectively