I downloaded the ebook.rar, ran AKC! with that on the Kindle, looked at the collections.json file, and immediately saw the problem.
I just had two statements in reverse order, so it was creating a partial file entry for a *.mbp file instead of ignoring those completely. I fixed the issue and the corrected file is now posted in place of the original. Please download and re-run that in order to confirm the fix.
Thanks for pointing that out. ^_^