Unknown by Jordan L. Hawk

Librarian Sebastian Rath and his lover Vesper Rune continue their search for the evil Books of the Bound. But in the meantime, the young men in Sebastian’s social circle start dying.
Though the deaths look like self-administered poisonings, Sebastian soon comes to realize a killer is at work. And not just any murderer, but one wielding the dark power of the Book of Bone.
To stop a madman, Sebastian must choose whether or not to embrace the unsettling powers gifted to him by the previous Books. Because if he fails to stop the killer, Ves could be the next target.