Thanks a lot. However I am not clear what it shows: the books that don't have isbn, or the ones where isbn has not been extracted yet.
As my library changes all the time with new books, books moved to other libraries, etc I never can remember for which books I already did the extract isbn. So I think I keeping on trying to extract isbn of books that I already tried before.
If an unsuccesful search would put something like 'not found' in the ISBN column, then I could do the search only for the books for which I never did a search.
I hope I managed to explain?
Thanks a lot!