The Illusion (Animorphs #33‪)‬

There is an Anti-morphing ray out there. It's purpose: to cause a person in morph to revert to his natural form. And the Yeerks have done a very good job in hiding it. Word from the Chee that the Yeerks are ready to test the weapon. Just one problem: They don't have a test subject. Yes, they could have Visser Three go into morph and them hit him with the ray. But Yeerk scientists say there's a definite chance that the ray will prove fatal. The Visser isn't volunteering.