Animorphs Book 28: The Experiment by K. A. Applegate

The Animorphs discover that the Yeerks are performing animal testing, and go to investigate. Ax has a brush with death that leaves him trembling. This book puts some important ideas in place but doesn't move a lot forward in the series arc.

This is one of the shortest Animorphs books, possibly the shortest one. I'll be looking for changes in Ax's behavior or personality after his most shocking brush with death yet, but he maintains his stoicism so completely that I don't know how quickly (or whether) any effects will appear in the series.