Out of Time by Monique Martin
Summary: Professor Simon Cross has spent his life searching for evidence of vampires and avoiding emotional entanglements. When a mysterious accident transports Simon and his assistant, Elizabeth West, back in time, Simon finally finds both the proof that he’s been looking for, and the romance that he hasn’t.
Simon and Elizabeth’s developing relationship is tested by demons real and imagined. In 1920s Manhattan, there are more than mobsters vying for power in the city’s speakeasies. When the local kingpin with a dark secret sets his sights on Elizabeth, day to day struggles become a fight for their very lives.