The book of darkness is a god book which is worn in the shield slot. It can be purchased from Jossik for 5,000 coins after completing Horror from the Deep.
Ancient books like the book of darkness are made once all four torn ancient pages are added to the book. These torn pages are obtained from Treasure Trails.
If lost, the book can be reclaimed from Jossik including the added pages for no cost. If you rather not travel to the lighthouse, Perdu will return the book for 14,000 coins.
+10 magic attack