Discovery. Katherine Applegate. 1998.

The blue box Elfangor used to Crete the Animorphs has been found by a kid named David. David ha 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 bo. At any cost.

But what should have been a simple plan ends up going horribly wrong. And David is on the run with the Animorphs and Ax. Now they have only two choices: turn David over to the Yeerks. Or make him the sixth Animorph.