Before she was known for releases like Crenshaw and the Newbery award winning The One and Only Ivan, Katherine Applegate brought the Animorphs saga to the world. The series shows Applegate's impressive skill as a writer, as she weaves an accessible sci-fi story of epic sweep and exciting action sequences with surprisingly sophisticated world-building and affecting character development. The narrative also touches on themes regarding the traumas of war (through a kid-friendly lens) and mankind's relationship to the environment. All of the above is spread across the series in books of digestible length. Appropriate for ages ~7+, but contains a level of depth that can engross readers of any age!