Who chooses which book is used and-or whether the decision will be deferred to a book - the GM, one or more players, or both combined?I'll add an example to illustrate my position.
Let's say that during the play it for some reason becomes important to know what gods, if any, northern ice orcs worship.
Some ways we can get this information are:
1) The GM decides.
2) The player decides.
3) A book tells us.
To me it is pretty clear that 1) gives the GM agency, 2) gives it to the player, whilst 3) really offers no meaningful agency to either. And I don't really think it much matters who reads the information from the book.