The Ancient Tome is found within the Chamber of Bone on Level 6 of the Catacombs in the original Diablo.
It is guarded by Horror Captains, Unseens and Horned Demons. Reading the tome bestows the ancient Guardian spell, and thus completes the quest.
Story Tomes — Dark Exile • Great Conflict • The Realms Beyond • Book of Blood • Mythical Book • Book of the Blind • Sin War • Tale of the Three • Wages of Sin are War • Binding of the Three • Tale of the Horadrim • The Black King • Steel Tome
Items — Ancient Tome • Fungal Tome
Hellfire Tomes — Journal: The Meeting • Journal: The Tirade • Journal: The End