After installing the Plugin I got the warning (shown below). When I tried to go to the link I got the message to say I was not authorised to access.
You are using WP Super Cache, in order to have ebook store working properly with it, you must add these lines to the "Ignore strings" section here. Exactly where it says "Add here strings (not a filename) that forces a page not to be cached.". The lines you must add are:
The administrator has disabled public write access.