Animorphs: #1 Invasion - K,A Applegate

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. So begins K.A. Applegate's epic series about five normal kids with a limitless amount of forms and abilities.