Totally agree on this. When I had my '88 Chevy Sprint, I had the original factory service manual, plus a gigantic book of parts diagrams with parts numbers and sizes of absolutely everything. It was an extremely valuable resource for working on that car, far better than the Haynes manual. They're not easy to find though for older, less common cars. I found mine on Craigslist 2000 miles away and ended up mailing the guy a $25 check (I felt comfortable with it because really, this would be an oddly specific scam, and it was only $25) and he mailed them out that week.
I still have them. I really should unload them so they can help another Sprint owner out.