Knight Eternal

A cursed girl trapped in a tower must choose between the prince who promises to free her and the immortal knight sworn to protect her in this romantic fantasy from Margaret Rogerson, the New York Times bestselling author of Sorcery of Thorns and Vespertine.
Ophelia has spent the past ten years locked away in a tower, haunted by a deadly curse, with only her pet dragonlet for company. She connects with the outside world through her books, her spyglass, and the experiments that have become her windows to everything she cannot reach.
In a kingdom where magic has faded to nearly nothing, a powerful curse like Ophelia's is a mystery that soon attracts the interest of the crown prince. Prince Tristan is handsome and charming, and his arrival allows Ophelia to descend from her tower for the first time in years. But as Tristan sweeps her up in a whirlwind courtship, she suspects he is keeping a secret from her about her curse—and finds herself growing ever closer to the knight he has assigned to keep her safe. Clad in dragonscale armor, bound by an unbreakable enchantment to serve the crown, the knight is feared by all who cross his path. Only Ophelia looks closely enough to glimpse the noble soul beneath, imprisoned by the cruelty of his vow.
Yet love cannot exist between them. For as Ophelia makes a discovery that could alter her fate and the fate of the whole kingdom, the knight's enchantment demands his uncompromising loyalty to Prince Tristan. Even if that forces him to betray his lady...and break her heart.