"The Andalite's Gift" by K. A. Applegate (Animorphs Megamorphs #1)

We never should have done it. But we needed a break. Some time off from the superhero stuff. A chance to act like normal kids. Well - as normal as four kids who can morph into animals, a boy trapped in a hawk's body, and a bright blue alien can be. Everything should have been cool. Now Rachel is missing. And there's this... this thing that's after us. But it's not up to me to tell the whole story. Tobias, Cassie, Marco, and Ax were there, too. Even Rachel has some info to add. So go ahead and check this out. And remember not to tell anyone what we're about to tell you. It could mean the difference between life and death. Or worse...