Animorphs Book 43: The Test by K. A. Applegate

Tobias battles his fear of his tormentor and the hunger of a Taxxon. This book deals with the aftereffects and continuing psychological damage caused by torture, specifically in a scenario where (due to morphing) no physical marks remain.

I really like the way that Tobias’s mental struggle over the Taxxon morph works to help him figure out how to deal with Taylor. I don’t remember if there is a lasting change in him after this book, but I hope there is, and I hope it’s positive.