The Ultima Underworld Clue Book: Mysteries of the Abyss is 64 pages long and contains game narrative and maps presented by the evil wizard Tyball, plus details on the game engine of Ultima Underworld. It originally retailed for USD$14.95 and was published in 1992. It was written by Aaron Allston.
This was the first hintbook to be assigned an ISBN code.
- Mysteries of the Abyss (PDF)