When a ghost of ancient Pompei is given the chance, he takes it to inhabit the body of a time travelling scientist, complicating an already taut situation. Rick has re-entered Mackenzie Cates' life and their passion has been reborn, but since neither is into menage a' trois, they need to get rid of the two thousand year old ghost living inside him, reunite said ghost and his doomed love in the far flung past, and save the world.
Good read. Distant future time travelers are trying to save the future by fixing the past. The hero [an archaeologist:] and heroine [a museum director:] are modern era people enrolled in the project because of their specialized knowledge... The time traveler and an intern at the museum are part of the project [and the hero/heroine of the next book:] which requires a trip to Pompeii on the day the volcano erupts. The book has good action, interesting characters, and believable premises. Order is not extremely important - I read the second one first and it wasn't a real problem.