The Thief is set in a fictional world based on Ancient Greece, but brought forward. The society has guns and cannons and has mostly forgotten the ancient gods, yet some aspects of that life remain. The main character, Gen, is in the king's prison. He is rescued by one of the king's advisors who needs his skills as a thief to help him steal an item from one of the neighboring countries. There are excellent characters in the book, and the conversations are delightful. Turner is excellent at creating natural, humorous conversations and situations.
As a child, this book was my favorite (I only read the first three). This time around it was actually my least favorite of the series, I think because I remembered more of the plot than with the others and that was very distracting. These books are easy to read, and they go fast. I highly recommend them for any age. My sister and I both reread them recently and had an hour-long discussion about them. They're that good.