Shadowspire — Jaye McKenna

Wytch Kings, Book 3
When the soon-to-be betrothed Prince Jaire of Altan meets a handsome ghost, he fears he might be losing his mind. But the ghost may hold the key to his kingdom's future, as well as the key to Jaire's heart.
Shy and bookish Prince Jaire of Altan is not particularly enthralled with the idea of marrying the simpering Lady Bria of Irilan, but Altan needs to renew its ties with the neighboring kingdom. For the sake of the alliance, Jaire is willing to do his duty, even if he would much rather marry a prince.
Sole survivor of the massacre now called the Irilan Rebellion, Prince Vayne of Irilan disappeared so long ago, no one even remembers his name. Trapped in the mythe to protect a secret that could change the balance of power in Skanda, Vayne has been fighting to retain his sanity for centuries.
When Jaire sees the ghost of a handsome young man dressed in fashions centuries old, he fears he might be losing his mind. But the ghost proves to be far more than a restless spirit, and might just hold the key to Altan's future, as well as the key to Jaire's heart.