The Weakness (Animorphs #37)

Jake has to attend a wedding out of town. No one has been designated to take over as leader of the Animorphs. But when Tobias tells the others that he's found the place where Visser Three currently feeds, there is a unanimous agreement on going after him. The kids aquire cheetah morphs to run the visser down. Unfortunately, there's a new player on the scene: an inspector sent by the Council of Thirteen to check on Visser Three's eternal problems. The inspector has a Garatron host body. Garatrons are the newest hosts: like Andalites, but with shocking speed

and no morphing technology. this Garatron-Controller outruns the Animorphs and Ax in their cheetah morphs, saving Visser Three. The kids soon realize this inspector may provide a way to do away with Visser Three. If they can convince the inspector that the visser is totally incompetent, totally unable to stop the 'Andalite bandits,' maybe the Visser will be recalled. Now a dispute breaks out: Who will run the mission in Jake's absence? Marco and Rachel are the main aspirants. Cassie takes herself out. Tobias and Ax bow out. In the end, Marco lets Rachel take control. Rachel stages a series of reckless Rachel raids to harass known Controllers, with the goal of making the Visser seem impotent. But Rachel's recklessness and impatience betray her when her failure to plan results in Cassie being captured and taken to the Yeerk pool. Rachel's confidence collapses. She learns about the flip side of leadership, the difficult choices Jake makes all the time. The kids rescue Cassie with a spectacular assault on the Yeerk pool. Rachel has succeeded, but her interest in leading is considerably diminished.