Magic of Faerun has some neat stuff in terms of spellbook protection. It'll cost you, but you can protect a spellbook from elements, make it waterproof, make it look like something else, make it more resistant and even spell-trap it.
There are lots of spells to protect it, though. Leomund's Chest, traps spells (Fire Trap, Sepia Snake Sigil, Secret Page...). If you're willing to go all-out, you could buy yourself a glove of storing, and put it there. The glove can still be stolen, though, but they'd have to figure out the password.