The Immortal City by Amy Kuivalainen

No one was willing to believe Penelope when she found evidence that Atlantis existed…and Alexis went out of his way to make sure of it. Now, a mysterious script has turned up at a murder scene, and Penelope might be their only chance to find the killer. Penelope thought her career in archaeology was over. Disgraced for trying to prove Atlantis existed, she's been made into a pariah in the academic community. With the police in Venice asking for assistance with a murder, Penelope will get her chance to redeem her name and prove her theories are true. Alexis, one of the few ancient survivors of Atlantis, has done everything he could to discredit Penelope because she was getting too close to the truth he's devoted his cursed life to protect. But with a serial killer revealing ancient mysteries no one should know about, Alexis will have to put aside his need for secrecy before anyone else gets murdered. Forced to work together to stop the attacks, Penelope is drawn by Alexis into a world of magic, danger, and passion that she could never have imagined. Neither one can escape the tragedies of the past, and only together can they protect their future. Get ready for an action-filled, romantic urban fantasy based in Venice, where danger lurks around every corner and magic is a deadly game. However, please note that this book contains adult themes, violence, and sex scenes.