Due to Alucard's sense of smell, he can identify a book on the shelf that contains traces of blood. If he opens the book, he'll find that a slot has been cut into the book and a small vial of blood is placed within.

Although it's not clear whose blood this belongs to, it's possible the blood will be required to complete the ritual or the potion needed to unlock the warding on Josselyn's door. They will still need to determine what role the blood plays, however.