For me it would be the article he and Mike Kone wrote on pricing. I can't tell
how many times I refer to that just to get an idea what a bike is (was) worth. Sure
the article is dated but it gives me a good idea of the bike.
the other would be this:
http://sheldonbrown.com/wheelbuild.html
without it I'd probably be doomed to buying boutique wheels or used ones off ebay.
Marty