The Ultima VII Clue Book: Key to the Black Gate (ISBN:0-929373-09-X) is 64 pages long and contains maps to all cities and dungeons, two different walkthroughs (one with hints and one with detailed help), and comprehensive tables of game statistics for Ultima VII concerning weapons, armour and all other goods, spell prices and healing. It was published in 1992 and originally retailed for USD $14.95.
In-universe, Anton, the apprentice of Alagner, with the help of the Xorinite Wisp and the Time Lord, wrote the book. The Wisp gives more general hints, while the Time Lord gives concrete help and advice. In real-life it was written by Andrew Morris.