Well, I'm totally OK with the #54 in terms of the ending of the Yeerk War (Yes, including Rachel's end. I know it is sad, but it would be not just silly but downright terrible and disgusting to conclude that war is not sad; at least she got to end it in a way that she totally loved. I just read Viss3r's post in the
http://animorphsforum.com/forum/index.php?topic=4192.msg442118#msg442118 thread and thought it summed it up amazingly).
I even think the descriptions of their lives afterwords was great epilogue material (Yes, the Jake/Cassie afterword material was sad, but again it is a powerful statement on the way that war changes you), the only place it really went wrong was with Ax: Becoming a prince and going about as a captain to be chasing remnants of the Yeerk Empire is a great way to end it; having what happened with him beyond that and the rest of the characters afterwords was just obnoxious though, it set up a story with no intention of concluding it; it felt like KA was afraid that with each book having its own buildup of an individual story to a climax that we wouldn't be willing to accept the final book as basically entirely falling action when that is basically exactly what the story that concludes the series should be.
It isn't even that peaceful sort of "The Grey Havens" ending that Lord of the Rings had, where after the action from this story falls you 'let the characters go' off on more adventures (or like that 'Riding off into the Sunset' of Westerns, same idea), because it sets up and gives too much of the next adventure but then says "Nope, I'm not going to tell you how it goes, what a dramatic way to end. Clever, don't you think?"