Animorphs Book 39: The Hidden by K. A. Applegate

The Animorphs are on the run from the Yeerks' morphing sensor. This book turns a slightly silly premise into a visceral horror and complex ethical dilemma when a bison accidentally gains the morphing technology. The usually body horror cws apply.

They're all dark, but now we have a new, more complex flavor. This also offers up the very worrisome implication that if any creature touches the cube they might become able to morph. They'll have to keep it very well sealed, for even more reasons now.

Cassie is getting a better sense of herself and what she wants, but she's becoming more conflicted about her role as the ethical one. She's circling around but hasn't yet voiced the thought that it's not better if someone else does something because she won't, but I can see the shape of that building, especially in her dynamic with Rachel.