Plot:
Earth now as another alien invasion, this time by tiny little people called Helmacrons. They believe Cassie stole their ship. Cassie thought it was just a toy, and gave it to charity. But, not taking that as an answer, the Helmacrons unleash their own weapon- the ability to convert matter into different sizes. So the Animorphs are shrunk, but, on the bright side, so is Visser Three . . .

Comments:
Since this is a Cassie-book, I'll try to look on the bright side. There are some interesting aspects on the reason behind the Yeerk's plan and on morphing that had previously not been revealed. Umm . . . there are a new group of aliens that have been introduced. Though it follows an interesting theory I once read (I believe it was Michael Crighton's The Andromeda Strain) that perhaps if there was life on another planet, it would advance by getting smaller and more efficient rather than getting larger and more complex as it has for Earth, there isn't much you can do with the "tiny aliens" thing. Of course it's one of those #-4 comedy books, but often instead of being funny it was just weird.