I read the last three books in one day. I had a lot of questions in regards to this one, but none of them seem very important now. This last bit of story telling in the series is really good. My only regret is that the writers (Applegate and all the ghost writers) didn't explore different groupings of characters more often.
We see that Rachel and Marco side together on votes often, but they have very little (if any) time spent together where they can develop any relationship beyond banter. I liked the one scene Marco and Cassie had together and think it would have been interesting to explore that further.
More under the cut.
In this book Ax is thinking something about Rachel (how he can't relate to her or something?) and I wish the writers had acknowledged that they were the two who stayed with David while he became a nothlit. That is something that's been referenced recently that ties the two of them together in a way that's separate from the rest of the group.
The books are getting bloodier. There is outright killing and the very real danger that at least one of our Animorphs will die in battle. I found it surprisingly emotional, especially when they're all standing on the train station platform before sending the bombs into the Yeerk pool.