Animorphs Book 10: The Android by K. A. Applegate

The Animorphs meet the Chee and discover that they aren't alone against the Yeerks on Earth. There's a very well-done interplay between pacifism and practical immortality, exploring the consequences of violence for beings who cannot forget.

Only 10 books in there are a lot of ptsd-type nightmares going on in this series and this book paints a horrifying picture of how much worse it would be for the Animorphs if they couldn't forget, if the memories wouldn't become dull with time.

Something I don't want these reviews to lose sight of is just how...dark... this whole series is. At this point they're canonically in middle school and they have the weight of the planet on their shoulders. I know series for kids can feel overblown because they're trying to capture how big everything feels when you're younger and don't have the tools to solve problems, but Animorphs really does a good job of keeping alive the idea that just being a kid is hard enough while still keeping all the planet-defending stuff in proportion.

This series can be easy to dismiss with all the goofy book covers, but they're dark, darker than I can easily convey while staying away from spoilers.

(This book also raises the nerve-wracking answer as to where the extra mass goes during small morphs, I just wish it answered where the extra mass came from when doing large morphs.)