I like this one a lot. Jake goes to the North Pole and becomes a Polar Bear to destroy the Yeerks next reviving place.
Marco, the other Animorphs, and Ax have managed to find out where the Yeerks are planning to build their next ground-based Kandrona. That's a good thing. The location is supposed to be somewhere around the North Pole. That's a bad thing. The Animorphs and Ax know that the Yeers are a "cold-blooded" species, but this is a little nuts! Who wants to be anywhere near the North Pole without Arctic morphs -- and wearing spandex?