Animorphs Book 37: The Weakness by K.A. Applegate

Jake is out of town and Rachel leads in his place. This book builds upon the differences between Rachel and Marco as leaders, highlighting Rachel's strength as a reactive tactical thinker and Marco's more careful planning.

I like the addition of the Garatron, it's nice to see more alien species. I don't remember if this one comes back, but I like the description of it.

The last chapter is so damn sad, I just... the feeling is building in these books that something has to give, something's going to break. The message that even if the Animorphs manage to win they'll never really be done is so unrelenting and heartbreaking.