Mortal Prey by John Sandford

I enjoyed this...Years ago, Lucas Davenport almost died at the hands of Clara Rinker, a pleasant, soft-spoken, low-key Southerner, and the best hitwoman in the business. Now retired and living in Mexico, she nearly dies herself when a sniper kills her boyfriend, the son of a local druglord, and while the boy’s father vows vengeance, Rinker knows something he doesn’t ... the boy wasn’t the target--she was--and now she’s going to have to disappear to find the killer herself.