I find Forte's GPS difficult to learn from because he does only demonstrate the techniques and the quality of the video is pretty low. I would suggest Jason England's Foundations DVDs to get started. The Unreal Work vol. 1 and 2 (available on dananddave.com) give you some great applications and demonstrations once you have the basic techniques down. For riffle stacking I would suggest the book By Forces Unseen, but only after you know the basics of it (preferably from England's download on this site).
I would pretty much second everything TheNinjew said. The B.F.U. shuffle work section has some of the best stacking problems I have ever seen. Really fun and addictive stuff! I would also mention Richard Turner DVDs. One that is a must is his DVD on Hops and Shifts. Probably the best all around resource for nullifying the cut which you will need to do in order to make your demos look really good. Ie. offer the deck to a spectator to cut.