Pleasure Exchange (Pleasure Games #3)
Cathryn Fox

Though Cat has kept her feelings hidden, Sam has been secretly stoking her inner fires for months. Now, with Sam's "wonder formula" tossed into the mix, their unleashed passion is off the charts, rocketing them together into the stratosphere of erotic ecstasy, no emotional strings attached.
In the name of science and journalistic investigation, suddenly nothing is taboo. But what happens when Sam and Cat's sizzling, sensual pleasure game starts to turn into the real thing?