"Animorphs 22 The Solution" by K. A. Applegate

David, the newest Animorph, is not what he appears. His need to control the other Animorphs and Ax is all he thinks about. And the things he does are starting to break up the group.

Rachel and the others know that time is running out. The newest battle against the Yeerks is the most important one yet. And it's not one that will wait. Winning this fight could mean slowing down the invasion. But no one knows what to do with David. Because the newest Animorph is more than just a little problem. He's deadly. . . .