Animorphs: Volume 2, The visitor

"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"--Provided by publisher.