Why does Books refuse to import from my Mac

I use Books on my iPad as an ereader and up 'till some weeks ago it refuses to import books it once had imported them without any problem whatsoever. The situation is as follows: I did an import of -about- 34 books I had stored in iCloud. Then I changed my mind and removed most of them from the library so there remained only 4 or five books, so I don't have to scroll through endless pages of the library. When I finished these books I wanted to import the next 5 books, but that didn't work. Books refuses to import books via File>Import and responds with Importing:0 of 1. Failed: 1

What's wrong with Books?