Animorphs. 2, The visitor

Summary/Review: "Rachel is still reeling from the news that Earth is secretly under attack by parasitic aliens, the Yeerks. Now she and her friends are the planet's only defense -- five kids who, purely by chance, stumbled onto a downed spacecraft and were given the power to morph into any animals they touch. The team's best lead is their assistant principal, Mr. Chapman, who is the human host to a high-ranking Yeerk official. It's not much, but Rachel's always been a daredevil, and she volunteers to infiltrate Chapman's home. Rachel is tough. She's fearless. But what she finds inside may be more than even she can handle."--