Megan Hart 
The Challenge (Mills & Boon Spice Briefs) [EPUB ebook] 

Want more than Fifty Shades? Mills & Boon have the answer with 12 Shades of Surrender! All 12 stories promise unforgettable romances with a sinful edge – surrender to pure reading pleasure… Katie Donato and Dean Manion are total opposites, but they both love sex, and a challenge. So when Katie tells Dean she thinks she could pleasure him better than he could ever satisfy her, he dares her to prove it-by sleeping together. Soon Katie and Dean are taking their friendship to places they never imagined…