Book of Magnus

Also refered to as the Book of the Thousand Sons, it is a massive tome of sorcery constructed by Magnus during the Great Crusade. The book currently resides in Magnus' library on the Planet of the Sorcerers, and a copy is said to be in the Black Library of Chaos. It is said that before the Horus Heresy, Ahriman was the keeper of the book, as he was the Cheif Librarian.