Try Borders or Barnes and Noble. When I was looking they had either hardcore advanced books or, and mostly, simplistic ones for Tiger. I was sorely disappointed.
It makes sense though if you think about it. OS X is an extremely powerful OS with an extremely simplistic UI. You get the best of both worlds and unfortunately, no one has written a book that really encompasses the power of the unix core with the simplicity and power of the UI (hotkeys, hacks, etc.), at least not that I'm aware of.