You've got something screwy set up on your Mac for zip files.
- Highlight the book, then click 'o' to open a finder window.
- Instead of double clicking, right click on the zip file.
- Click 'get info'
- Find the 'Open With' area, click the dropdown, and select 'Archive Utility'
- Click the 'Change All' button just below that - it may prompt for your password/ask if you're sure at that point, can't remember.
If you don't have Archive Utility for some reason you can install some other app beforehand and specify that - Stuffit Expander is generally pretty good, as is the Unarchiver.