Warriors: Shimmers of Hope: Book One: A Broken Path

Littlepool is a young warrior of RainClan. She has only been a warrior for a moon. It was her first gathering as a warrior, and she had made friends with Nightpaw of BerryClan. Littlepool was excited to tell him the news, and also found out that he was a warrior too! His warrior name was Nightfang.
After the gathering, in the middle of the night, a lot of commotion is happening in the camp. And it turns out that Plumpaw, the medicine cat apprentice, had a message from StarClan. Plumpaw says that Dapplestar, The previous RainClan leader, had told her that Littlepool, a warrior from BerryClan and the medicine cat of HollyClan, would be going on a quest to find an ancient cave, known as the Cave of Shimmering Stars.
After bumping into him, Littlepool soon discovers that the warrior from the message was Nightfang, and that he had been told by his medicine cat, too. So they met up with Acornberry of HollyClan, and began their quest to find the Cave of Shimmering Stars.