Mark of the Thief (Scholastic, Feb. 2015) didn't quite live up to my hopes (which were perhaps unrealistically high), but it's a good read none the less. The inclusion of a griffin as the main character's companion animal, and the magic of the Roman (ex-Greek) gods add lots to the kid appeal, and making it one to offer the multitudues of somewhat younger Percy Jackson fans.
Nic is a slave in the salt mines, worried about keeping his little sister safe. There's not much he himself can do for her, as his life is not his own. And this is proved definitively one day when he's ordered to enter a cavern in the salt mines that has been the death of everyone else who's tried to enter it. The cavern contains a great magical treasure--an amulet that once belonged to Julius Ceaser himself! It is filled with magic from the gods, and Nic takes it for his own. But griffin guarding the caverns treasures wounds Nic, marking him with more magic....magic that lets him communicate with her (to some extent). With help from the griffin and the magic amulet, Nic escapes from the cave...only to be plunged into worse troubles.
Because of the magic he has found, Nic is now a player in a power struggle to control the Roman Empire. The most powerful general, the most powerful Praetors, and the emperor himself are now all very anxious to relieve Nic of his magic and take it themselves....and Nic, confused, hungry, and wounded, must struggle to stay alive until he can decide what to make of his new destiny....and find his little sister, and make a place of safety for the two of them.
From the Colosseum to the sewers of Rome, from great estates to the temples of the gods, Nic stays barely ahead of those who would use him for their own ends....And there's never a dull moment. Although fans of The False Prince might find Nic overly reminiscent of that book's hero at first, he comes into his own, and character interest is provided by his relationship with a girl named Aurelia, who variously befriends, saves, and betrays him, and who has secrets of her own.....
I felt that this book took perhaps too long to really get at the meat of its magic--where it comes from, why Nic has it, and what he might do with it. But there was action aplenty, the bonus griffin relationship was a nice touch, and the whole set up promises good things in the next books!
Here's another review at Ms. Yingling Reads.