The Illusion (Animorphs #33‪)‬

Tobias and the other Animorphs already have a few problems: saving the world, keeping their identities secret, finishing their homework. Now they have one more thing to add to the list. The Yeerks have discovered a way to cause a person to demorph. And with this new device Visser Three plans on capturing "the Andalite bandits."Now what? The Animorphs realize they have to get rid of the device -- or be captured by the Yeerks. And the risk of getting caught is very high. That's when Tobias comes up with the only possible plan. Someone has to acquire Ax...