Animorphs - The Reaction

In this book, Rachel, one of five kids who can turn into animals, has an allergy. Not any old allergy - an allergy that happens to people who can turn into animals. The allergy makes her turn into an animal without even making herself do it. What if she turns into an animal in front of her parents?  What if she morphs into an animal while the bad guy is watching her? 

I liked this book because it was tense. It had no negative sides.