The Decision (Animorphs #18)

Ax and the Animorphs are about to have a huge problem. It starts when they decide to morph mosquitoes in order to slip by some unsuspecting Yeerks, and ends with them stuck in Zero-space with no idea how they got there, no way back to Earth . . . and no oxygen. Luckily, an Andalite scout ship finds them before it's too late. But now Ax is finally with his own people. And he doesn't know if he ever wants to go back to Earth.