The 21st Birthday by James Patterson and Maxine Paetro

In this thrilling novel from a #1 New York Times bestselling author, Detective Lindsay Boxer takes a vow to protect a young woman from a serial killer long enough to see her twenty-first birthday. When young wife and mother Tara Burke goes missing with her baby girl, all eyes are on her husband, Lucas. He paints her not as a missing person but a wayward wife—until a gruesome piece of evidence turns the investigation criminal. While Chronicle reporter Cindy Thomas pursues the story and M.E. Claire Washburn harbors theories that run counter to the SFPD’s, ADA Yuki Castellano sizes Lucas up as a textbook domestic offender . . . who suddenly puts forward an unexpected suspect. If what Lucas tells law enforcement has even a grain of truth, there isn't a woman in the state of California who's safe from the reach of an unspeakable threat.