I know that at least the Library, and probably other parts of ADOM, are unknowns in terms of what you can expect to see in an average game, and don't lend themselves easily to individual testing because of the sheer time required to get a character to and through the rift, particularly a character (or more than one? do we even know if Library results are guaranteed for non-wizards?) of each class. As such, I'd like to suggest something simple - whenever you play a character that goes to the library, post what results you got in terms of books/scrolls (this room was books, this was scrolls, et cetera) in this thread. We can do the same for anything else that's unknown and not easily testable by individuals (for example, I get a tension room in the lower left of the CoC Graveyard every time I fully explore it; is this guaranteed, or am I just extraordinarily unlucky?) and eventually get knowledge that wouldn't otherwise be available.
I'm working on a paladin and archer currently, and I'll mosey them both over to the Rift after they close the gate to see what there is to see.