Ok now that I've watched the episode, the book is called the Darkhold. It is the book of the damned aka the book of sins. It was created before the year 10,000 BC by an Elder God called Chthon. It can't be destroyed as a Saint and Merlin together tried to destroy it and couldn't. This book was used by the Darkholders (the black magic users that used the Darkhold) to create vampires.
There's a huge history that goes through Atlantis and much later on Dracula. Ghost Rider had something to do with it at some point. So it might be that they're going to introduce vampire's now, or maybe Chthon is the "devil" that gave Ghost Rider his powers in this version of the Marvel Universe. Could be going a few ways. Dr Strange does come into possession of the Darkhold at some point and uses it to destroy vampires, so it does tie in.