The Quickie by James Patterson

When Lauren Stillwell discovers her husband leaving a hotel room with another woman, she decides to beat him at his own game. But while she's sneaking around, her husband is hatching a plan of his own. After a torrid quickie with a co-worker, Lauren hears a struggle outside her window and looks out just in time to see her husband loading her lover's limp body into the trunk of a car. When the body shows up in a pool of shallow water, she races to the scene of the crime. But Lauren Stillwell is no regular wronged woman. She's a NYC cop—and she's just been assigned to this case. Unable to tell anyone what she saw and unwilling to turn her husband in, Lauren is paralyzed by a secret that will tear her life apart. But as she attempts to point fingers away from her husband, she uncovers something shocking: her husband didn't have an affair—what he did was far worse than she could have ever imagined.

A gripping story of secrets and infidelities that begins where Adrian Lyne's movie Unfaithful leaves off, The Quickie will have readers' hearts pounding to the very last page.