The Secret (Animorphs #9)
There's something pretty weird going on in the woods behind Cassie's house. The place where Ax and Tobias call home. It seems the Yeerks have figured out one very important thing: Andalites cannot survive without a feeding ground. Visser Three knows the "Andalite bandits" don't feed where he does, so there can only be one other place.
Now Cassie, Marco, Jake, Rachel, Tobias, and Ax have to figure out a way to stop a bogus logging camp. Because if Visser Three finds Ax in the woods, nothing will stop him from finding the Animorphs...