Rising Storm (Warriors)

Rising Storm is the fourth book in the first "Warriors" series: "Warriors: The Prophecies Begin", written by Erin Hunter. It takes place in a fictional forest populated by four groups of feral cats that call themselves Clans. There are four Clans: ThunderClan, ShadowClan, WindClan, and RiverClan. The cats that live in these Clans call themselves warriors. 
Plot.
Fireheart, now second in command to Bluestar, the leader of ThunderClan, must deal with Bluestar's failing mental state, as well as the rest of the Clan's needs. He also has to train his troublesome apprentice, Cloudpaw. Cloudpaw, having been born outside of the Clans, fails to follow the rules that the Clans live by, which gets him into trouble. He eventually tries to beg for food from humans, which results in him being captured and taken away.
A cat named Ravenpaw finds Cloudpaw in a human house far away from the Clans. Ravenpaw comes to tell Fireheart where Cloudpaw is, and so he, Fireheart, and another cat called Sandstorm go to rescue Cloudpaw.
Later on, Fireheart realizes ThunderClan territory is on fire, and he warns the Clan. His early warning meant that most of the Clan could escape in time. However, Yellowfang, an old friend of Fireheart's, and several old cats die in the fire. ThunderClan takes shelter in RiverClan territory for a while, before returning to their own territory, to find it burnt and ruined. A few days later, Fireheart takes ThunderClan to a Gathering (a night when all four Clans meet to share news) and discovers that his old enemy, Tigerclaw is now the leader of ShadowClan.