The Departure — K.A. Applegate

We can't tell you who we are. Or where we live. It's too risky, and we've got to be careful. Really careful. So we don't trust anyone. Because they find us ... well we just won't let them find us.

The thing you should know is that everyone is in really big trouble. Yeah. Even you.

Cassie's had it. After the last mission, she realizes she's getting tired of missions. Tired of battles. Tired of being an Animorph. She decides that she just can't do it anymore. So she quits.

But that's not the worst that's happened. It seems a human-Controller named Karen followed Cassie after the last run-in with the Yeerks. And she knows Cassie is an Andalite . . . or human. Either way, if she exposes Cassie, it's all over. No more Cassie. No more Animorphs. No more planet Earth. . . .