The Art Collector (Demons Among Us) by Katelyn Brehm

A quiet art historian. An irresistible demon. And an insatiable hunger neither can escape. Madelyn Frye enjoys her simple, drama-free life, even if it means there's an empty seat next to her on the couch at night. But when her firm lands a lucrative contract to appraise the paintings of an infamous mansion with a storied past, her comfortable routine is upended by the handsome and mysterious millionaire who owns it. For over a century, Thomas von Dreiss has resigned himself to a solitary fate, refusing to let his hunger transform him into a monster. But he never expected the art appraiser who shows up at his door to test his unshakable restraint. Her brilliant mind and kind heart rekindle long forgotten hopes and set his carnal needs aflame. Will Madelyn risk shattering her safe and predictable world to be with Thomas? And if she does, will Thomas's dark secret destroy their chance at a happily ever after?