Shadowbound - McMaster Bec

When a powerful relic goes missing from a secret society that dabbles in the occult, Miss Ianthe Martin is charged with finding it at all costs. She needs help, but all clues point to someone on the inside being the thief. The only sorcerer she knows that can't possibly be involved, is the very man she saw locked in Bedlam a year ago... 

The mad, bad, dangerous Earl of Rathbourne. 

When the seductive Miss Martin appears in his Bedlam cell, Rathbourne fears he's finally lost his mind. The devilish sorceress played a hand in his incarceration, and now she comes asking for help? Perhaps she should begin by begging for mercy... 

But Ianthe's offer of freedom is one he can't refuse, although he has a clause of his own to add. She may bind him with her power–the only way to still the demons haunting him–but for every day spent under her command, the nights will be his... to wreak delicious revenge on her willing flesh.