J, I don't know what kind of folders you want to make, but I bought
The Tactical Folding Knife: A Study of the Anatomy and Construction of the Liner-Locked Folder by Bob Terzuola It is a pretty good book on Liner-locked folders, and gave good details on the geometry of the lock and and most of the problems that can go wrong with that type of locking system. I still haven't made one yet, I'm gonna concentrate on fixed blades for right now. I also bought Peter Atwoods video on Linear-locks and It was pretty good seeing how the lock was cut, and actually seeing the process, Another book I bought was Custom Knifemaking By Tim McCreight. I think it is mainly for beginners has a lot of heat treating stuff ect but I does show the workings of several folding knife designs except for Liner-Lock.
Hope that helps. It was also nice meeting you at the Columbus show.