My most useful tool for working on vintage bicycles isn't literally a tool. It's my Sutherland's Manual.
I would highly recommend it to anyone that works on vintage lightweight bicycles, if you don't already own one of the older editions.
The original orange one (from 1974) is pretty basic, but even the (green) second edition is better. I probably use my (green, three-ring bound) 1990 Fifth Edition the most, as it includes most of the stuff that I encounter on a daily basis, except multi-speed hubs. There is an excellent section on cotterless crank fit issues, for example, that graphically explains the various BB spindle differences way better than I can....
Greg Parker
Ann d'Arbor, Michigan