Blood in the water : a Gregor Demarkian novel / Jane Haddam.

Enhanced descriptions from Syndetics:

Gregor Demarkian returns in a mindbending case of death and disappearance amongst the wealthy suburban elite.

In Waldorf Pines, a very rich, gated suburb of Philadelphia, ostentation and pretension are the order of the day. But even by the local standards, Martha Heydrich is a stone cold pain. She's the stay-at-home wife of a very rich husband, drives a pink sports car everywhere and is on all the prominent local committees. She's fake, into everybody's business and is rumored to be having an affair with a local teenager, Michael Platte. One morning she seemingly vanishes from her house and later that night her husband Arthur returns home to find the pool house ablaze. Once the fire is extinguished, the police discover two bodies--one is Michael Platte and the other, too damaged to be recognizable, is
presumed to be Martha Heydrich. The police think they know what happened--that Arthur killed his wife and her lover over the affair. But then the DNA results come back and the second body isn't Martha's at all, it is an unknown man. With their theory in tatters, and Martha nowhere to be found, the police to turn to ex-FBI agent Gregor Demarkian to help them unravel this most puzzling of cases.