The Underground (Animorphs #17)

What's tasty, good for you, and only takes sixty seconds to make? Oatmeal. And because of a strange reaction to the Yeerk's physiology, it's making the would-be invaders of Earth more than a little crazy. Now Rachel and the other Animorphs have a new weapon against the Yeerks. Sounds good, right? Wrong. Because using this advantage against the Yeerks has dire consequences for their human hosts. In order to avoid hurting innocent people, the Animorphs will have to make another trip to the Yeerk pool, where they barely escaped with their lives. The Animorphs have taken chances before and been very lucky. But this time their luck may be about to run out.