The (Phone-calling) Horse Book. I'm not overly fond of Cassie books for various reasons. But this time it's not her fault she was saddled with this funny, but ultimately pointless installment of the Animorphs saga. The best part is undoubtedly the fact that all the characters, from the main six to the controllers to Visser Three himself, are aware this is a stupid, pointless story, and say so several times. 
 (Speaking of Visser Three, holy OOCness and Villain Decay, Batman.)