Animorphs Book 24: The Suspicion by K. A. Applegate

The scale of the Animorphs' battle with Visser Three changes and they have to size up battle with a new alien species: the Helmacrons. After a lot of very heavy drama, this is a little lighter but no less deadly serious. A good interlude.

I really really enjoy the Helmacrons as a species and as a neat bit of world-building in the series. When I was a kid, this book taught me the word "fungible" and the associated concept in a truly memorable way. I also remain fond of the Helmacrons' criteria for leadership, even though the ceremony is a bit sharp.