A few years ago, another instructor and I had students launch a "Professional and Technical Writing" entry in Wikibooks. Now, ithe project is stale. 

http://en.wikibooks.org/wiki/Professional_and_Technical_Writing

This last year (2011-12), I attempted to get my students in a technical writing course to evalute the project and reboot the project. I wasn't successful, as the students argued that nobody knows of Wikibooks and there are plenty of free writing books / content available throught iTunesU and iBooks.