"Cross" by bestselling mystery/suspense author James Patterson.

From the publisher:

Alex Cross faces the deadliest psychopath of them all -- his wife's killer …

Alex Cross was a rising star in the Washington, D.C., Police Department when an unknown man gunned down his wife in front of him. But his need for vengeance was placed on hold as he raised his children himself.

Years later, Alex leaves the FBI to work as a full-time psychologist. His life finally feels like it's in order -- until Cross's former partner, John Sampson, gives him a chilling phone call. Sampson is tracking a Georgetown serial rapist whose brutal modus operandi includes threatening his victims with terrifying photos. Now Cross and Sampson need the help of the rape victims to stop the predator, but the women refuse to talk.

When the case triggers a connection to his wife's death, Alex may finally have a chance to catch the murderer. Is this a chance for justice at long last? Or the culminating scene in his own deadly obsession?