Lots of people recommend the park tool website for how-tos. Their "Park Big Blue Book of Bicycle Repair" seems to have much of the same information, plus more. I have the second edition. Very nice book - but note that it's not really all that big. You can't always (or want to) be at a computer while working so this book is good to pick up. It's small enough that you can port it around easily if you're so inclined. The Zinn book is much larger, and uses illustration. I have, like, and use both, but I prefer the photos of the Park book over the Zinn book's illustrations.

It would be nice if these books were available in a binder-style option so the pages could stay open and lay flat (and I realize this would probably cost more).