Librarian Skeleton

This one does not appear because you are in the dungeon, it appears because there are placed books nearby. The game scans a square of tiles around a candidate point looking for one, takes the closest, and then checks that the book sits far enough from you to be well off your screen. If nothing qualifies, nothing spawns. The book does not need a shelf, does not need dungeon walls behind it, and does not need to be anywhere near where you eventually meet the thing. That cuts both ways. If you are building inside the dungeon and keep getting jumped, the shelves you left standing a screen or two over are the reason. If you want the drops, do the opposite: plant a stack of books, then walk far enough off that they stop counting as on screen.

It blinks in, waits a beat, throws a short volley at you, and vanishes again. The books are not conjured out of nothing. It grabs a copy off an actual placed book near it, which is why a projectile sometimes rises from a shelf across the room instead of from the skeleton, and the shelf is never emptied by this. Each book floats upward before it launches and does no damage during that part, so the windup is a generous tell once you know to watch for it. If it cannot find a book to lift, it makes one beside itself instead.

Landing any hit cancels whatever it was about to throw and pushes its next teleport back, so continuous pressure keeps it blinking around the room without ever completing an attack. Ignore it and it settles into its own rhythm and works through you instead. The books pass straight through blocks, so backing behind a wall achieves nothing and closing the distance is the correct answer. It also ignores the rule that pins other dungeon casters in front of unsafe walls, so a room you sealed off and repapered will not keep it out.