Obtained in the Pits of Inferno Quest, found in a chest south of the Plains of Havoc temple. If you need a new one, Oldrak will reset your Tible quest flag for 1,000 gp allowing you to obtain it again. The book is found near the Knightwatch Tower (here. Note that you need either the Levitate spell or 3 Parcels to go up the mountain.
Grants access to the Pits of Inferno and thus required for the Pits of Inferno Quest. Used to pass through the magic walls below Goshnar's grave.
Spoiler ends here.