The Chateau by Tiffany Reisz

As the Jack-of-All-Wicked-Trades for a secretive French military intelligence agency, 24-year-old Lieutenant Kingsley Boissonneault has done it all—spied, lied, and killed under orders. But his latest assignment is quite out of the ordinary. His commanding officer’s nephew has disappeared inside a sex cult, and Kingsley has been tasked with bringing him home to safety. The cult’s holy book is Story of O, the infamous French novel of extreme sado-masochism.

Their château is a looking-glass world where women reign and men are their willing slaves. Or are they willing? It’s Kingsley’s mission to find out. Once inside the château, however, Kingsley quickly falls under the erotic spell cast by the enigmatic Madame, a woman of wisdom, power, and beauty. She offers Kingsley the one thing he’s always wanted. But the price? Giving up forever the only person he’s ever loved.