Prince's Gambit: Captive Prince Book Two by C. S. Pacat

Never had he wanted something this badly, and held it in his hands knowing that tomorrow it would be gone, traded for the high cliffs of Ios, and the uncertain future across the border, the chance to stand before his brother, to ask him for all the answers that no longer seemed important. A kingdom, or this.

To get what you want, you have to know exactly how much you are willing to give up.