What an interesting series. I don’t think that there are many other books that are quite like this one, where when you read it, time just flies by. Most books in the series have about 35% of the book leading up to the climax, and the climax is just so, so good. I absolutely loved it. In particular, I enjoyed the 3rd book, The King of Attolia and the last book, Return of the Thief. The 2nd book, The Queen of Attolia had a lot of short climaxes, with very interesting, and heart-stopping moments.
In the series, Eugenides, the Thief of Eddis is tasked with stealing a stone that grants immortality, gets reunited with all his family and cousins in Eddis, a country next to Sounis. He also gets captured by the Queen of Attolia, a neighboring country, and suffers a grievous injury. He starts a war, ends a war, becomes a King. And finally, he becomes Annex over Sounis, Eddis and Attolia. I really, really loved Return of the Thief, it had so many funny and interesting parts, I just read it over and over again. Same goes with The King of Attolia. Honestly, the worst out of all of them (which is still really good) is Thick as Thieves.
As a series, I would rate it for ages 10+, and 4.5 stars out of 5, because for me, the books are so different compared to each other, although it is quite minor, but individually, they are awesome. Also, I highly recommend reading them in order, otherwise it will seem extremely confusing.