Kushiel's Dart by Jacqueline Carey

Phedre no Delaunay is an anguisette: touched by a god to experience pain and pleasure as one. Though she only sets out to be the finest courtesan in the realm, fate, Anafiel Delaunay, and Kushiel have other plans for her. More than just a courtesan, Phedre also becomes a spy... and finds herself in the middle of political intrigues and plots far more vast than she ever imagined. A brilliant villain and a plot to destroy her homeland force Phedre to become more than she ever thought she could; and it's only the beginning.