The answer (animorphs #53)

The battle is still raging, and it's out-and-out war. Jake and the other Animorphs have found a way to slow the Yeerks down, but now the Yeerks have decided the best way to win is to just destroy everything and everyone who gets in their way.Just when things look hopeless, Jake and the others discover help where they least expected: Taxxons. Creatures so foul and heartless, it's hard to believe that they're capable of having any feelings. But Jake and the others soon discover that the Taxxon resistance has its own problems with the Yeerks. And for once, it looks like Earth may have a chance....