The downloaded file needs to be unzipped into its separate files.
It can be difficult to work out where Win10 has put them.
One sequence that seems to work is a follows.

1. Click on DOWNLOAD link to get the zip file sent to your "download" folder.

2. Navigate to your download folder aand double click on the .zip file.

3. An extraction/edit window should appear.

4. Use "Ctrl A" to select 'everything'

5. Click on the "Extract" button on the control bar.

6. A "Copy to" window will appear.

7. Click on the little down arrow in the address window
and choose C.\Users\Owner\EDIT_DRIVE\
or some other place you know you can find...

8. Click OK

9. Navigate to C.\Users\Owner\EDIT_DRIVE\ ( or your own chosen place)
and you will hopefilly find the html cover file there and the folder of contents.

10. The Ebooks are designed to start by clicking on the .htm cover file.

11. Click on the .htm cover file and the cover should appear with options.

[ Relocate both file and folder to your library if that is what you want ]