I can't remember ever intentionally targeting spellbooks, but if they happen to be in the blast zone then so be it.
That said, usually spellbooks are in a backpack. For things like AoE damage we give the backpack a save first, and if it makes it then everything inside is considered to have survived as well. But if the backpack fails, then everything inside is exposed and has to save for itself.
Fireproof red-dragon-skin spellbook covers are rare but really sought after. They don't, however, help much if the spellbook gets drowned or has to save vs lightning, acid, etc.