The Return (Animorphs #48)

David, the traitorous Animorph, is back. When the Animorphs decided to give another human being the power to morph, it was one of their biggest mistakes. David tried to destroy them all -- and almost succeeded. Rachel and the others thought they'd seen the last of him when they trapped David as a nothlit, leaving him on a deserted island to live out his days as a rat... They were wrong. Rachel knows what a threat David is. He's cunning and dangerous. Worst of all, he knows their secrets. Now he's captured Rachel as part of his plan. David wants power. He wants money. But more than anything, he wants revenge...