Captivated by You — Sherrilyn Kenyon

The second sexy story in her series featuring the Bureau of American Defense. Rhea Stevenson is a "BAD" agent who has just been handed her most challenging assignment. She must go undercover as a dominatrix to bring in a deadly terrorist. The only upside is that her fellow agent, "Ace" Krux, whom she has long desired, is her training partner. Almost immediately the roles of master and slave are wonderfully blurred and Rhea and Ace unleash a wealth of hidden desire.