iBooks Author can't open old .iba document

[Poki]

I just tried to continue work on a book I started at the beginning of the year. I haven't opened it for a few months now, so all my time machine backups are from the same file.

When I try to open it, the blue bar comes, fills, and it then says "This file cannot be opened". I unzipped it and everything seems to be there, even the index.xml. So what can I do to open it? I can't imagine the document got corrupted since it worked the last time I edited it.