If I can stretch the definition of "book" to include RPG books, I'd like to state my nitpicky dislike of vague secret door descriptions. I would love it if dungeon designers would put a bit more effort into describing how their secret doors work. Do they swing on hinges like a normal door or rotate around like the fireplace secret door in
Indiana Jones and the Last Crusade or do they slide into a cavity in the wall or ceiling? Also, do they require a hidden button, or do you pull a latch disguised as a book on a shelf or twist a torch sconce or what?
Instead, it's more often just a DC to spot the thing and then it just "opens" to reveal the next room or passage or whatever. And then there will be a bit about how it "opens easily" from the other side.
Yeah, sure, I can fill in the details myself, but it would be nice if the designers would do it for me.