Warriors: Power Of Three: Dark River

Description for WARRIOR:  POWER OF THREE:  DARK RIVER by Erin Hunter:

THERE WILL BE THREE. KIN OF YOUR KIN...WHO HOLD THE POWER OF THE STARS IN THEIR PAWS

Lionpaw, Hpollypaw, and Jaypaw —grandchildren of the Clan leader Firestar—are thriving as the newest ThunderClan apprentices. Yet with their new responsibilities come new experiences, new power and secret dangers.

Lionpaw makes a friend—and a discovery—that must be kept hidden: Jaypaw learns a secret that could benefit ThunderClan only by bringing harm to others; and Hollypaw knows something that could avert a battle, if only she could convince the rest of her Clan. Prophecy looms, ominous forces threaten to tear them apart, and each young cat discovers darkness: in the past, in the Clans and in themselves.

This is #2 in Warriors:  Power Of Three Series.