At this moment they only spawn in dungeons and stronghold libraries, while enchanted books spawn in desert temples, dungeons, jungle temples, mineshafts, pillager outposts, all stronghold chests (though much more common in libraries), underwater ruins and woodland mansions, in addition to being a fishing drop. It feels kinda bad because their appearance rate in dungeons is pretty low and the only other place that can actually make them is strongholds. Tomes should spawn in more structures.