Silent Joe by T. Jefferson Parker

Scarred for life by a brutal father, Joe Trona found a safe haven and a loving childhood in the home of the couple who adopted him. Now he spends his days as a deputy for the Orange County sheriff's department and his nights as a driver and aide to Will Trona, the influential politician who rescued him from the Hillside Children's Home. An expert in firearms and the martial arts, Joe has been backing Will up for a long time. Still, his skill isn't enough to keep Will alive, and when his father is killed right in front of Joe's eyes, the young deputy vows to avenge him. But first he must find out how the kidnapping of a tycoon's daughter, a scam to line the pockets of Will Trona's political enemies, the murder of two Guatemalan immigrants, the unholy activities of a charismatic minister who's a close friend of the Trona family, and the strange alliance of two rival L.A. gangs are connected to Will's death.