Harbor Island / Carla Neggers.

Enhanced descriptions from Syndetics:

In this vivid and suspenseful addition to her widely acclaimed Sharpe & Donovan series, New York Times bestselling author Carla Neggers takes readers on a heart-stopping journey from Boston to Ireland to the rocky coast of Maine

Emma Sharpe, granddaughter of world-renowned art detective Wendell Sharpe, is a handpicked member of a small Boston-based FBI team. For the past decade Emma and her grandfather have been trailing an elusive serial art thief. The first heist was in Ireland, where an ancient Celtic cross was stolen. Now the Sharpes receive a replica of the cross after every new theft--reminding them of their continued failure to capture their prey.

When Emma receives a message that leads her to the body of a woman on a small island in Boston Harbor, she finds the victim holding a small, cross-inscribed stone--one she recognizes all too well. Emma's fiancé, FBI deep-cover agent Colin Donovan, is troubled that she's gone off to the island alone, especially given the deadly turn the thief has taken. But as they dig deeper they are certain there is more to this murder than meets the eye.

As the danger escalates, Emma and Colin must also face do-or-die questions about their relationship. While there's no doubt they are in love, can they give their hearts and souls to their work and have anything left for each other? There's one thing Emma and Colin definitely agree on: before they can focus on their future, they must outwit one of the smartest, most ruthless killers they've ever encountered.