Best one yet for me as bathroom material was EliteII - Frontier. Recently destroyed in a water leakage accident, but it was worth a small burial ceremony. 3 books, including one that had a lot of short story fiction in it, plus a fairly extensive map of the closest region in space. Awesome!
Did the 'water leakage accident' have something to do with the fact that you were using it as 'bathroom material'....?
Anyways, I too love having an in-depth manual to go along with my games (at least my favorite ones like TES), but I wouldn't be surprised if we're moving closer and closer to having digital-only versions of the manuals for all games, even for consoles.