Into the Wild by Erin Hunter

Into the Wild, is the juvenile novel that kicks off the popular Warrior cats series by Erin Hunter.

This series has inspired a lot of fan content: name generators, youtube videos and fan art. 

Maybe its so popular because it involves a heroic journey. Rusty makes a journey into the unknown, the wild forest beyond his house. He is initially met with derision from the feral cats who live there. Then, Bluestar recognizes his talents and chooses him to join them.

Rusty is accepted as an apprentice but must prove his mettle if he is to become a warrior. Along the way, Rusty, renamed Firepaw, has several crises.

Firepaw wonders if its always right to follow the warrior code. Should he have mercy on a pitiful cat like Yellowfang who has gone rogue? Should he tell Bluestar the dreadful secret he learns from Ravenpaw?

The warrior code is in jeopary in other ways. The four cat clans that have co-existed peacefully for years. Recently, however, the Shadow clan has pushed the Wind clan off their territory. 

In the end, its up to a brave apprentice, Firepaw, and his companions to set things right.