The Familiar (Animorphs #41)

Jake seems to have grown up overnight–literally. When he went to asleep he was his usual kid-self, and when he woke up, about ten years had flown by! What's worse, the Yeerk invasion of Earth has succeeded. The newly-adult Jake searches for Ax and the other Animorphs to find a way out of the horrific future–but he soon learns they won't be much help. They've all been killed or infested.

Luckily, Jake locates Cassie, who's part of a human-yeerk conspiracy movement to brutally conquer earth. When he attempts to help Cassie's rebel group though, he's captured by the new Visser Three. And now it looks like Jake's fate has been sealed. Because not only is the new visser more powerful than the old one, he also has a smart, cunning host armed with intimate knowledge of how the Animorphs fight. Marco.

It all seems too horrible to be true. And there are things that just don't add up. That's when Jake begins to wonder if it's all just a really bad dream. Because if it is, he'll wake up and things will be back to "normal." If not, Jake is as good as dead....