If anyone is interested, I'm writing a rewrite of Book 48, since I was a little disappointed. David is one of my personal favourites, and I just thought that, if he was going to return, he should return as a potential threat. I mean, the Animorphs are getting closer and closer to the end of the invasion by this point, and David's return could jeopardize that progress.
IF anyone is interested, you can find it on FFN.
Animorphs 48: the Return, by Tobias Mason Park...
You know, if you're interested....
Currently four chapters.