I had the same problem when I was new. Nobody would tell me what the heck a ML dungeon was.
So here is the short and sweet answer.
You can find scrolls in:
Labyrinth, which spawns minotaurs and Miasma
The Citadel, which spawns humans and Travesty
Sanctuary, which spawns mostly repond creatures
Bedlam, which spawns undead
Twisted Weald, which spawns changelings, spiders and cu sidhe
Blighted Grove, which spawns animals and Lady Mel
Palace of Paroxysmus, which spawns balrons, poison elementals, grizzles and of course Paroxysmus
Prism of Light, which spawns crystal things and shimmering effusion
Painted Cave, which spawn Troglodytes, Lurg and Grobu
You find most scrolls on named monsters (Lurg, Grobu, Miasma, Swoop, Silk, Irk, etc etc), but I've found a rare scroll on other creatures in the same dungeons, the more powerful, the more likely to find a scroll. My books come almost exclusively from Miasma, where I usually find one or two scrolls per run, which for me lasts about 15-20 minutes. Some scrolls are harder to find than others. For example, I haven't seen a nature's fury drop for me in quite some time, and nearly every other drop seems to be wildfire.