Thunder by Erin Hunter (Warriors - A Starless Clan: Book 4)

TO SAVE HER CLAN'S FUTURE, SHE MUST LOOK TO ITS DISTANT PAST

Frostpaw was told to trust no cat, but a brutal attack has left her gravely injured, and she'll need to depend on someone. When help arrives in the form of a young warrior, she convinces him to protect her as she follows StarClan's cryptic directions toward the roots of RiverClan, where Riverstar has promised she'll find the answers she's been so desperately seeking.

Back at the lake, the tension between ShadowClan and RiverClan might be easing, but the peace is fragile. Until StarClan's chosen leader is found, the slightest push could lead the Clans back into outright war.