Captive Prince, by S.U. Pacat.
This is a mystery, a detective novel, a suspense, a thriller, a military novel, a horror to those who are queasy, a Fantasy book, a heavily kinky erotica...and a romance. The author NEVER writes down to the reader, the reader must rise to the clues. Damen is our point of view character, and Laurent is his completely enigmatic nemesis. Laurent is brutally, savagely cruel. There are two major motivations Laurent has for this behavior, and the attentive reader will spot the first motivation soon after we first meet him, while Damen remains oblivious to it almost until the last page. The second motivation that inspires almost all of Laurent's behavior is only revealed to the reader slowly over the course of the entire book. Damen never even discovers this basic fact that shapes everything about Laurent. He remains happily oblivious to it. And in a coup of writing skill, Pacant actually never overtly states this fact about Laurant to the reader, either.
But to a reader sensitive to the topic, this second motivator to Laurent's behaviour is made excruciatingly clear by the last chapter. This motivator is behind every of Laurent's most unforgivable depths, and the highest leaps of ideals he can reach for beyond them. It is behind his every weakness, his every mistake, and his locked down spun-glass sensitivity.
Speaking of the last page (and treating the two volumes as one book), Pacant also respects the reader by not over-explaining the last reveal, instead believing that the reader will put it all together. This is a twist based book, and is now in my favourite three twist-based stories, along with The Golden Age trilogy, by John C Wright, and the Night Angel trilogy by Brent Weeks. I didn't want The Captive Prince to be over, and so I actually read the Author's Acknowledgements page for once, and was rewarded by a hidden Epilogue up the end.
I cannot remember another book that has teased out the Delayed Gratification trope to such exquisite lengths. This book is so well written that I believe it deserves to be broken out of every genre limitation. I defy an intelligent reader not to be haunted by this novel, no matter how far outside their usual genre or comfort level the Captive Prince is.