Cliffhanger. Totally. But you know the humans won the war, sort of. The Yeerks also got a decent deal out of it.

It was pretty realistic, really -- nobody really 'won' the war, everyone suffered, but the war did come to an end and then WHAM, something new started up. I think. I really wish I knew if the Animorphs actually survived the new thing they went rushing off into, but her letter makes the ending a lot more palatable to me.