Jenna by Susan Ward

Jena Garret lives a high-octane life. She’s the best weapon against evil our government has. Beautiful enough to be a centerfold. Trained in weapons and hand-to-hand combat, she can hold her own with even the most deadly Special Forces operator, though her most accomplished skills of espionage are in the bedroom, with both men and women.
Jena was Holly’s best friend when they started high school. By graduation, Holly avoided Jena at all cost. Their friendship had become confusing, the emotions so complex that Holly couldn’t even risk being near Jena.
Seven years later, unhappily married Holly is thrown together with Jena at a bachelorette party. And Jenna hasn’t changed. She’s even more unnerving.