My Lady Jane by Cynthia Hand

Edward (long live the king) is the King of England. He’s also dying, which is inconvenient, as he’s only sixteen and he’d much rather be planning for his first kiss than considering who will inherit his crown…

Jane (reads too many books) is Edward’s cousin, and far more interested in books than romance. Unfortunately for Jane, Edward has arranged to marry her off to secure the line of succession. And there’s something a little odd about her intended…

Gifford (call him G) is a horse. That is, he’s an Eðian (eth-y-un, for the uninitiated). Every day at dawn he becomes a noble chestnut steed—but then he wakes at dusk with a mouthful of hay. It’s all very undignified.

The plot thickens as Edward, Jane, and G are drawn into a dangerous conspiracy. With the fate of the kingdom at stake, our heroes will have to engage in some conspiring of their own. But can they pull off their plan before it’s off with their heads?