The Weakness (Animorphs #37) by Katherine Applegate

When Jake has to leave town for a family thing, the other Animorphs and Ax figure life will pretty much be normal until he gets back. But when Tobias discovers Visser Three's newest feeding place, the kids decide they have to check it out.

But there's no assigned leader. Now Rachel, Tobias, Cassie, Marco, and Ax have another problem: Who will be the best one for the job? Because being in charge isn't just guts and glory. It's about making decisions that will mean the difference between life . . . and death.