I cleared out under-Ulcaster and didn't find the tome the ghost was beebling about. I searched carefully, I thought, with Imoen detecting. Did I miss it, or is there another layer? I suspect the slab next to the stairs leads to a subbasement, but even a potion of hill-giant strength didn't move it. Perhaps some spell will open it? I found no other likely entrances with Imoen detecting against the edge of the plateau.

SPOILER
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There are no sub-levels to the Ulcaster school. One thing I noticed about this dungeon is that the passages that run NW to SW can be hidden. Chances are you haven't explored as thoroughly as you thought. It's difficult to explain. The book should look like an average book. good luck.

Link in my sig
Chapter 13
BG1 section
Chapters 2-3