The sales brochures from 1973, '74, and '75 are still around and show up on eBay and the web. They are a great source. Also the ads from the '70's are here on this site (jerrydoe.com) and if you concentrate, you will see there is a bike behind that girl! :lol:
I also find Roy Bacon's book has some good pictures, but be careful to choose the one's that are obviously taken at the factory or dealer as there are several non-standard photos.