Beyond the Code — Erin Hunter

With Leafstar's encouragement, Sol, a former kittypet, has joined the ranks of SkyClan. He dreams of becoming a great warrior, and promises to be helpful and brave. But Leafstar soon begins to wonder if Sol respects the rules of the warrior code. And when Sol's actions lead to a perilous disaster for SkyClan, Leafstar has to figure out whether he can be trusted -- before it's too late.