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 will need either the Levitate spell or 3 Parcels to go up the mountain.
Spoiler ends here.