I've just uploaded a new version. See the first message in this thread.
Enhancement: 1.0.2 now checks for the presence of an existing zip file, and gives the user the option to replace the file or stop. And the old file really does get replaced, as it's moved to the trash before the new one is created. Previously, files left over from the old file could end up being left in the new file.
Originally Posted by pdurrant
Here's an AppleScript application that does all the hard work for you - just drag the folder of files onto the Applescript application, and your epub will be created.