Animorphs Series54 books in series
The Invasion Publisher's Summary
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.