Warriors Books by Erin Hunter
There is peace at last between the warrior Clans, and Firestar is proud of the strength and unity of the cats he leads in ThunderClan. All four forest Clans are thriving, training new warriors and keeping their boundaries without conflict.
But Firestar's dreams are haunted by wailing cats fleeing a terrible disaster. With unexpected help from an old kittypet friend, he discovers a shocking secret: StarClan, the warrior ancestors who guide his paw steps, have lied to him.
Firestar is faced with the hardest decision of his life. Can he really turn his back on the forest that has become his home and embark on a perilous quest to discover a dark truth—one that has been buried beyond the memory of living cats? Whatever he finds at the end of his journey, he knows that nothing can ever be the same again.