The website "The Isthmian Archive" might be of interest to some :-
http://www.isthmianarchive.co.uk/index.htmlAlthough there does not seem to be any section devoted to referees, the "Isthmian News Section" contains scans of 44 editions of the league's monthly newspaper, the first edition of which was published in December 1977 :-
http://www.isthmianarchive.co.uk/page10.htmlMeanwhile, the "Bulletins Section" contains scans of the league's weekly bulletin beginning with the 1988/89 season :-
http://www.isthmianarchive.co.uk/page5.htmlThe website is still being added to. At the moment, one significant omission is the league's annual handbooks.
There is currently no easy way to isolate the documents contained on this site which include information relating to match officials.