ibook author .iba file "does not have a valid format"

Hi there

We are working with a publishers on creation of an iBook. The have supplied an .iba file for us to open, but when trying to open (either by double-clicking, or by going to 'open' in iBook Author) we get an error message that stated '"Filename.iba" could not be read. The document does not have a valid format'.

I have tried changing the filename extension to '.zip' to attempt to uncompress the file, but got this error message "Unable to expand "Filename.zip" into desktop. (Error 2 - No such file or directory.)"

Any ideas how we can go about reliably sharing .iba files for proofing/amending purposes?

Thanks