Forest of Secrets by Erin Hunter

Allegiances are shifting among the Clans of warrior cats that roam the forest. With tensions so delicately balanced, former friends can become enemies overnight, and some cats are willing to kill to get what they want.
Fireheart is determined to find out the truth about the mysterious death of brave ThunderClan warrior Redtail. But as he searches for answers, he uncovers secrets that some believe would be better left hidden.