Animorphs: The Visitor

The second book in the Animorphs series follows Rachel as she discovers her best friend Melissa's father has been taken over by a Yeerk, an alien parasite that controls human hosts. Rachel must navigate the moral complexity of potentially having to harm someone she cares about while using her grizzly bear morphing ability to fight the secret alien invasion. The story explores themes of friendship, moral ambiguity in warfare, and the psychological toll of being a teenage soldier in a hidden conflict.