The Book of Cain is an official Blizzard licensed product – indeed its contents were produced by a number of the core individuals who have been working on the Diablo series since its inception. The book provides fans with deeper insight into the Diablo game universe.
The Book of Cain is stylised as a tome of writings collected over the ages by Deckard “Stay a while and listen” Cain, a character whom Diablo series fans will know as their ever-present guide and mentor. Cain is the last survivour of the order of Horadrim, a collective of mages who trapped the three Prime Evils and guarded their Soulstone prisons.
So, he probably knows what he’s talking about.
The book is presented as Cain’s gift to his niece, Leah. To explain who Leah is, we need a quick recap on Diablo lore.