My dad once said to me, "You give a lot of books three stars." I do.
I'm giving this one four and a half stars mostly for nostalgia factor. If you took that away though it's still a solid four star Animorphs book. Definitely one of Cassie's best (Melinda Metz is a good ghostwriter and it's unfortunate that she only ghostwrote two books).
There were some things that didn't make sense to me this time around. I'll discuss them under the cut.
Why did Cassie have to infest Mr Tidwell? He could have carried her in his pocket and just released her into the Yeerk pool. She's been there before. She knew what to expect. She gains nothing by being in his head really.
It doesn't seem like good security to take Tobias to the school dance. They say no one recognizes him anymore, but there's always the chance someone will remember him. And wasn't Chapman asking about him not too long ago?
Cassie morphs directly in the Yeerk pool which seems risky. We know that Yeerks echolocate (or something like it), so one of them could have "seen" Cassie going through her natural human body before morphing to Osprey. Do we really believe that with all that commotion in the Yeerk pool, none of the Yeerks were bothering to "look" around? I don't know what the alternate solution is though.
I wish the story had gone more into Cassie's first encounter with Aftran. Cassie says that she made the right decision at the time, but really whether the decision was "right" or not is changing all the time. Shortly after book 19 it was the right decision because everything worked out for Karen and the Animorphs. But now if Cassie fails, it could have been the "wrong" decision since Aftran will be giving up all the secrets about the Yeerk resistance and the Animorphs. I think there was more to explore there and wish the book had gone a little deeper into the consequences of Cassie's decisions.