The Winner’s Curse by Marie Rutkoski

As a general’s daughter in a vast empire that revels in war and enslaves those it conquers, seventeen-year-old Kestrel has two choices: she can join the military or get married. But Kestrel has other intentions.

One day, she is startled to find a kindred spirit in a young slave up for auction. Arin’s eyes seem to defy everything and everyone. Following her instinct, Kestrel buys him—with unexpected consequences. It’s not long before she has to hide her growing love for Arin.

But he, too, has a secret, and Kestrel quickly learns that the price she paid for a fellow human is much higher than she ever could have imagined.