I wonder why there aren't more of these stat boost books, and why they're all at the same place instead of hidden throughout the game.

The real "secrets" in this game i.e. loot boxes, are boring and not worth your time due to randomized loot.

Stock Trader #1: This is a stock exchange! There's no money you can steal!
Bane: Really? Then why are you people here?
-The Dark Knight Rises