There is lots of interesting information and plans on building several different stringed instruments on this page. I particularly like the inexpensive banjo and the instructions for converting a mini guitar to a mandolin.

There's not much detail on the actual construction, but there's plenty of info on how to do it. Unfortunately there's no email link. :( Oh, and the site prevents you from using the 'back' button, at least in Mozilla, that's a bit irritating.