I would recommend the wiki, or a document at
http://docs.google.com/.
I'd love to create and maintain a "
LaTeX" document of this book. LaTeX docs are just plain text files with special markup (somewhat similar to HTML.) You define the structure of your document (e.g. \chapter{Name of Chapter 1} My text...) and the system figures out the rest (i.e. styles, hyphenation, chapter numbering, indices, cross referencing etc.) You can, of course, customize fonts and other stuff if you like. It's the best way to create high quality PDF files.
Problem is, synchronizing with your updates will be difficult. There's no central document, where only certain editors have access.