The Thief - Megan Whalen Turner (Book 1 in the Queen's Thief series)

Eugenides, can steal anything - or so he says. Then his boasting lands him in the king's prison, and his chances of escape look slim.

So when the king's magus invites him on a seemingly impossible quest to steal a legendary object and win back his freedom, Gen in no position to refuse. The magus has plans for his king and his country. But Gen has plans of his own.