"Warriors #4: Rising Storm", (Warriors: The Prophecies Begin) by Erin Hunter (Author), Dave Stevenson (Illustrator)

Fire alone can save our clan...

Fireheart's traitorous enemy, Tigerclaw, has been vanquished and exiled from ThunderClan; but Fireheart can't shake the feeling that he's lurking out there in the forest, waiting for the chance to strike.
That's not the only problem facing the young ThunderClan deputy in these blazing summer months, as he struggles to handle ominous omens, a disrespectful apprentice with a shocking secret, and a devastated Clan leader who is a shell of her former self.

Meanwhile, the forest gets hotter and hotter... and everyone braces for the coming storm...