To get the book enter Barnabus's Cave and enter the topmost teleporter which will teleport you to a crypt. This crypt is filled with level 22 Undead Warriors, Shaman's etc, go through the rooms till you reach the last room and there is the book on the ground, the left side, before the chest.
Take the book and return to Igor at your tower for your reward.
Rewards: 4500 exp, 520 gold, Skill Book.
Plus a choice of either 1125 exp or 520 gold