Animorphs Book 36: The Mutation by K. A. Applegate

The Mutation brings a new level of revulsion and body horror to a series already full of both. It features a kind of believable and deeply terrifying portrayal of Atlantis. Jake is trying to be a good leader but it's getting really tough.

This whole series is dark, but for the last few books we've been treated to new and horrifying types of darkness. It's not so much that it's getting darker, as that it's spreading out to the sides in a pool of creepy stuff.