Forest of Secrets (Warriors)

Forest of Secrets is the third book in the first "Warriors" series: "Warriors: The Prophecies Begin". It was written by Erin Hunter. It was published on 14 October 2003.
"Forest of Secrets" takes place in a fictional forest populated by feral cats who live in groups called Clans. There are four Clans: ThunderClan, ShadowClan, WindClan, and RiverClan. The cats that live in these Clans call themselves warriors.
Plot.
Fireheart, a ThunderClan warrior, looks for a way to expose Tigerclaw, deputy (second-in-command) of ThunderClan, for being a traitor. Graystripe, Fireheart's best friend, is in love with a RiverClan warrior, Silverstream, despite relationships between cats of different Clans being against the rules the Clan cats live by.
Silverstream and Graystripe have kits, Featherkit and Stormkit, but Silverstream dies giving birth to them. Graystripe takes the kits to ThunderClan, where he is exposed for having a forbidden relationship. RiverClan is angry about this, and wants to take the kits back.
Fireheart and Graystripe find Ravenpaw, Tigerclaw's former apprentice, and get the true story of what happened in the battle where Redtail, the former deputy of ThunderClan, died. They find out that Tigerclaw's story of what happened that day was a lie, and that Tigerclaw killed Redtail. Fireheart tries to tell Bluestar about this, but she doesn't believe him.
Eventually, Tigerclaw brings a group of former ShadowClam cats into the camp, and tries to kill Bluestar during the confusion of the battle. Fireheart rescues Bluestar, and Tigerclaw is banished from ThunderClan. Fireheart is made deputy in his place, and Graystripe takes his kits to RiverClan. Bluestar's mental state begins to fail because of Tigerclaw's betrayal.