Warriors: Into the Wild by Erin Hunter

This book is about wild cats that roam the forests outside of civilizations and the story of a house cat who joins them and ultimately earns their respect, and eventually their loyalty. Erin Hunter does an excellent job of capturing the mood of the experiances he faces as he meets the different clans and becomes part of the noblest of all, Thunder Clan. The others are Wind Clan, River Clan, and the devious Shadow Clan. They are the fiends of the forest, except for the humans and other large animals.