"Take heed, and bear witness to the truths that lie within, for they are the last legacy of the Horadrim."— Intro text(src)
The Book of Cain is an object that is used to identify all unidentified items in a player's inventory with a single 5-second cast. It can be found in every town of Diablo III, right next to the stash chest.
The Book was originally written by Deckard Cain. It incorporated texts from various sources, as well as original writings and research. These were focused on Sanctuary and its history/geography, Heaven and Hell and their respective denizens, and the Prophecy of the End of Days.
Before Deckard Cain is killed by Maghda, he can perform the same action as the book if spoken to. Before patch 2.0., the Book of Cain was used to identify multiple rare items at once. After the identification of rare items was canceled, it now serves only for very rare occasions of player having more than one unidentified legendary item in their inventory, as identifying just one item takes exactly as much time as using the Book.