Force of Nature by Suzanne Brockmann

Florida private investigator and ex-cop Ric Alvarado's life is spiraling out of control. His beautiful new Girl Friday, Annie Dugan, is far more interested in fieldwork than filing, and despite Ric's best efforts to ignore the attraction, sparks are flying between them. Then one of Ric's clients turns femme fatale and tries to gun down an innocent man. Thanks to quick thinking and even quicker reflexes, Ric comes to the rescue, only to learn he's done a very good deed for some very bad people. Suddenly Ric finds himself deep undercover with Annie, working for notorious crime boss Gordon Burns. One mistake from Ric's painfully inexperienced partner and they're both dead.