I have found a terrific resource for repair, setup and other information on guitars, banjos, and mandos. They guy who runs "Gryphon Stringed Instruments" has this site
It's mostly oriented towards guitar but has info on other instruments. He sells all sorts of stringed instruments through Gryphon but this site is more for DIY folks- strings, setups, necks, capos, etc. Lots of great info here.
If you're interested in esoterica such as the tonal characteristics of different woods and how the grain affects sound, you'll find it here among other things.