The discovery (animorphs #20)

The blue box Elfangor used to create the Animorphs has been found by a kid named David. David has no idea what he has -- or what it can do. But Marco does. And when he sees David with it, he knows the Animorphs have to get that box. At any cost.But what should have been a simple plan ends up going horribly wrong, leaving David is on the run with the Animorphs. Now they have only two choices: turn David over to the Yeerks or make him an Animorph.Part one of a trilogy.