Animorphs Book 32: The Separation by K. A. Applegate

Rachel has an accident in starfish morph and has to come to terms with herself. I like this exploration of Rachel's personality. It could have been cheesy, but it crafts a story where both parts of her have something she needs

The dynamics with the other Animorphs play out in several very interesting ways. Specifically I'm thinking of her approaches to Marco and Tobias (including their reactions), and how Cassie is the first to figure out what's going on. Erek's role is strange here, the plot needs him around but he seems more confused by humans than makes sense for someone his age.