Not sure what "serious" means in this context, but Megan Whalen Turner's "The Thief" fits your criteria, and is a really, really good book.
Quote:
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.
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http://meganwhalenturner.org/NovelsThief.html
It's the first in a series, but it works well as stand-alone, too. The culture is inspired by ancient Greece, the tech level is renaissance-ish, and the plot and characters are really, really good.