This 1997 Newberry Honor story by Megan Whalen Turner has a wicked twisty plot and lots of descriptions of what people in Ancient Greece would have worn and eaten.
No plot giveaways here! The twists are too good for that. But here's a short premise: Gen was imprisoned for stealing, and now the King and the King's magus [wise man/scholar] has appointed him to commit espionage, and to steal something from the neighboring country.
A fun read. I liked it. It takes quite a long time for the story to spin its way around all the twists, and the ending felt kind of rushed - almost to the point of being half-thought and half-baked - but overall, fun.
The main character is humorously self-centered, which, I suppose is a good indication that he's a teenager; his age isn't specifically given, neither is his physical description. Which is refreshing. I like imagining what a character would look like solely based on his, well, actual character.