The Winner's Kiss by Marie Rutkoski

Arin thinks he is over Kestrel, but he is lying to himself. He thinks she has betrayed him and her words of love were false.   He has been told that Kestrel has married a prince from another country, and they are honeymooning in some faraway land.  Little does he know that Kestrel has been sent to a work/concentration camp because her father found out that she was in love with Arin, and was sympathetic to his country's cause.

War is eminent, and both Arin and Kestrel realize the need to fight the Emperor and his quest for land and power.  Kestrel must first escape from the work camp, and she cannot do that without strength.  Her daily rations are tainted with drugs that keep her from thinking clearly.  Will Arin find out about the lies he has been told before it is too late for Kestrel?  Will they ever be together again?

This book is a Young Adult Fantasy and is the final book in a trilogy called the Winner's Trilogy.  There is an element of romance, action, danger, and good versus evil.

Start with Book 1 (of course) and learn about how Arin and Kestrel meet when she purchases him at the slave auction.  Follow their adventures until the final battle.