Not entirely sure if I maybe just somehow missed the Tome in the chest in the Mausoleum when I first did the dungeon way back when, but I don't *think* I missed it. So I got a Rumor Boss Lich which spawned in the Mausoleum, went and killed him, then decided I might as well clean out the rest of the dungeon for loot and so forth, and there was the Tome in the chest! I remember clearing out the dungeon completely before, I'm almost 100% sure I got the Tome the first time around, I'll try to see if I ever get another Lich spawning there if it happens again.