What's the URL for the book on our website?
My guess is that it's a PDF file, and they do not automatically appear in the library on the iOS app.
You'd need to get the Adobe version of the file if it is a PDF and get it into the app. It sounds as though you have been doing that via the blue Adobe button on the "I'm Reading" list on the website. It is possible there is a limit on the number of times you can download the file from there, which might be the reason for the error.
The best thing to do would be to contact our customer care team by logging a ticket here:
http://kobo.frontlinesvc.com/app/lanpag