The Test (Animorphs, No. 43)

Taylor, Tobias' arch-nemesis and the Yeerk that once infested his mind, holding him, his memories, his every thought and feeling hostage,has kidnapped him once again. Only this time, she's going to let him go-after he's listened to her proposition...Taylor is now part of the Yeerk peace faction and tells the Animorphs of a plan to eliminate the Visser and the Yeerk pool, a plan which only the Animorphs can carry out. This would be a major victory in their war against the Yeerks, but it also means morphing to the creature with the most uncontrollable instincts-the Taxxon. Can Tobias prevent the morph from turning him against his friends...?