The Reunion (Animorphs #30)

Marco's mom is back. But she's not Visser One anymore. Marco's not even sure if she's still a Controller, but he's determined to find out. No matter what it takes. No matter what might happen. Marco wants his mom back. Jake and the other Animorphs realize that Marco is under some serious stress. And that the situation with his mom could very well jeopardize everything they've worked for. Now they also have to wonder if Marco will be the one to give away the secret of the Animorphs.