The Animorphs Series
by Katherine Applegate (Author)
The thing you should know is that everyone is in really big trouble. Yeah. Even you.
The Earth is being invaded, but no one knows about it. When Jake, Rachel, Tobias, Cassie, and Marco stumble upon a downed alien spaceship and its dying pilot, they're given an incredible power — they can transform into any animal they touch. With it, they become Animorphs, the unlikely champions in a secret war for the planet. And the enemies they're fighting could be anyone, even the people closest to them.