The Revelation (Animorphs #45)

Things were already really weird. Fighting aliens. Battling to have Earth. And still trying to be normal. Marco and the other Animorphs are almost used to it. But things are changing. The Yeerk invasion of Earth started out passively, secretly. But now, everything seems to be stepped up. Even Marco's father is talking about some top secret project at his job. Something about discovering Zero-space...

Marco doesn't even know whether his father is a Controller. But he does know he's not going to let the Yeerks win this one. They've already got his mother. And Marco will do anything it takes to save his father. Anything...