The Prince’s Harem by Megan Derr

The worthiness of the royal family to rule is displayed in their harems: those who willingly surrender their lives to serve as concubines, entertaining, pleasing and attending the royal to whom they are sworn, forbidden to do anything else, or even touch another person without permission.
Prince Aradishir is approaching his twenty-fifth birthday, but has not a single concubine to his name. Though he longs to prove he is as worthy of his position as the rest of his family, and has never lacked for offers, he has yet to find even a single man who makes him want to ask the vital question: would you be mine?
Until he crosses paths first with an illegally kept slave rescued from a deadly beating one night… a misfit soldier desperate to find where he truly belongs… and a daring thief known for achieving the seemingly impossible, but always secretly wishing for the truly impossible.