This is an ancient book, said to have been written by Corvus Corax himself back in the days of Horus Heresy. It is a very detailed and deep manual that describes creation and modification of Space Marines. It is said, that when Raven Guard Legion was decimated by the Traitors, Primarch started searching for a way to create marines faster, so that he could help defend the Imperium. His research was a success, he managed to breed Astartes that were faster, stronger, smarter and more capable in every regard than any other marine ever created. However these results came at a frightening costs - many of them mutated, went mad or simply lost their will to fight. Some believe, that using the secrets written in Raven's Tome was the main thing that later drove Primarch to go to exile and leave for the Eye of Terror, he just couldn't live with the guilt of knowing what he has done. However this knowledge is sacred and we can only pray that Corvus' sons will never again need to use them.