The Fungal Tome is, unlike the many other books and tomes found in the original Diablo, a quest item and unreadable.
It is obtained after looting the body of a fallen adventurer somewhere on the ninth level of the dungeon (the caves) beneath Tristram. Picking it up initiates the Black Mushroom quest after which players need to speak with Adria.
It is said to be filled with all kinds of reagents used in potions and elixirs, most notably the Black Mushroom. It is unclear who the author of this book is.
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