They are certainly better than all the novels that came later but they are still fanfiction even if officially licensed and published fanfiction.
I mean its like good fanfic versus rotten fanfic that is just printed solely to make money.
Zahn only had the first 3 films to work from in their original forms except for star wars which had been retitled episode IV a new hope.
He had the VHS tapes that came out in the 1980's to work from. Probably in full screen.