Into the Wild (Warriors)

Into the Wild is the first book in the "Warriors: The Prophecies Begin" series. It was written by Victoria Holmes and Kate Cary under the pen name of Erin Hunter. It was published on 21 January 2003.
Plot summary.
Rusty is an ordinary house cat who dreams of living in the wild. One night, he decides to explore the nearby forest. Before he goes, he is warned by his friend, Smudge, that the forest is filled with dangerous wild cats. Rusty goes into the forest anyway, and meets the leader of ThunderClan, Bluestar. Bluestar is the leader of one of the four wild cat Clans living in the nearby forest. He decides to accept her invitation to join her Clan and becomes Firepaw, an apprentice of ThunderClan. The other Clan cats are initially reluctant to accept Firepaw because of his house cat origins, but he slowly earns a place in the Clan.
Firepaw becomes friends with the apprentices Graypaw and Ravenpaw, and falls in love with the Clan's medicine cat (a Clan healer with spiritual abilities) Spottedleaf. He learns to hunt and fight, and he learns about Clan traditions. He finds another friend in Yellowfang, an old and grumpy medicine cat who was exiled from ShadowClan and taken into ThunderClan.
The four Clans, ThunderClan, WindClan, ShadowClan, and RiverClan, fight with each other to occupy hunting grounds. In a battle that takes place right before Firepaw joins ThunderClan, the ThunderClan deputy (second-in-command), Redtail, dies. Firepaw later learns from Ravenpaw that the former deputy was not killed by an enemy Clan, but by Tigerclaw, a ThunderClan warrior. Tigerclaw killed Redtail because he wanted to be deputy instead. Tigerclaw knows that Ravenpaw saw him kill Redtail, so Firepaw and Graypaw help Ravenpaw escape the forest to live on a farm, because Tigerclaw plans to kill Ravenpaw for knowing his secret.
After driving out WindClan for more hunting territory, ShadowClan tries to drive out ThunderClan, leading to a battle. The new deputy, Lionheart, is killed during the fight, and Tigerclaw is named deputy. After this, Clawface of ShadowClan kills Spottedleaf, and steals some ThunderClan kits. Everyone believes that it was Yellowfang who took the kits, because she disappeared the moment the kits were discovered missing. Firepaw and Graypaw, follow Yellowfang's scent to ShadowClan territory, to find out the truth about Spottedleaf and the missing kits. Together, with some help from within ShadowClan, they drive out Brokenstar, the evil leader of ShadowClan. Yellowfang, having proved her loyalty, becomes ThunderClan's new medicine cat, while Firepaw and Graypaw are made warriors (full members of the Clan) and receive their warrior names, Fireheart and Graystripe.