It depends on which part of the SF area you're in. If you're in Marin I'd recommend 'Mountain Biking Marin' which covers all of Marin and Mt Tam. There's even a website for the book : http://www.marintrails.com/biking/index.html. Most of the buses in Marin have bike racks on the front and the majority of the trailheads can be reached via bus and a short road ride or just a longer road ride, the exceptions being the ones in the more remote north-western part of marin.

I don't really know much about East, South or the rest of the North bay. West of the bay isn't very good for mountain biking