@miquele - I will have to take a look at some point to see what the actual response from Goodreads contains. On the surface it does indeed sound like a bug.
@paola - you can create empty books in that Goodreads Sync dialog for any that you don't have matches for. Then just add a tag or custom column if you want to identify them that way. There are various ways people do it - some create a special dummy text file that has a .PAPER extension so it shows up as a format. I just use a short tag. Or you could use a custom column, up to you. Once you have those book records created in calibre you can sync goodreads against calibre as you would for any other book type.