I found several blogs that recommend a fantasy fiction series for young adults by Megan Whalen Turner, The Queen's Thief series. The books in order are The Thief, The Queen of Attolia, The King of Attolia, and A Conspiracy of Kings. The Thief, won a Newbery Honor award in 1997.
About The Thief from the author's website:
The most powerful advisor to the King of Sounis is the magus. He's not a wizard, he's a scholar, an aging solider, not a thief. When he needs something stolen, he pulls a young thief from the King's prison to do the job for him.
Gen is a thief and proud of it. When his bragging lands him behind bars he has one chance to win his freedom—journey to a neighboring kingdom with the magus, find a legendary stone called Hamiathes's Gift and steal it.
The magus has plans for his King and his country. Gen has plans of his own.