Amrûnril
Hero
I don't know if the spellbook plays any special role in the module you're playing, but to answer the question more generally, not every treasure owned by a defeated enemy is going to be useful to every party. A party of DEX characters and Barbarians won't have any use for a suit of plate mail taken from a defeated fighter, and a party of spellcasters and throwing weapon users won't have much use for a magical bow. You can debate whether to make substitutions for such treasures or whether to allow them to be easily traded or sold, but there's no reason to treat a spellbook in a party with no wizards (or tome warlocks) as an inherently different issue.