Lincoln Electric has a lot of good info on their website (including student packs and the like) as well. If you really want to learn, look around at your local community colleges/trade schools and sign up for a class. They will have them from absolute beginner to pros looking for certification and what not. I signed up at a local community college a few years ago, and have gone back 3 times since as I added tools (MIG and TIG) and it's nice to learn the RIGHT way the FIRST time, rather than thru 'experience'!
I just finished a class in art welding (
) where I told the insructor all I was really interested in was aluminum welding techniques, so he let me work on that the whole time. Dan (the teacher) is a mega-certified welder who builds airframes and the like, and he taught me a TON of stuff that I dont know if I would EVER have picked up on my own. In this area, the cost was 150 bucks for the class, and about 75 in materials for the six weeks. Now I can start my aluminum car, with confidence!