Just go buy the most current book on body work you can find at a "new" book store to get started.Used books are ok but they are dated and may not cover newer processes so if you buy a used book bear that in mind.
Most of the basic techniques are still the same as they always were.
The filler and paint processes keep changing. On the newer cars that use high strength(high carbon) steel for the sheet metal you should not oxy acetylene weld on the panels because it messes up the steel composition.Mig & Tig are the preferred methods because the heat affected zone on the panel is smaller.
Many of the manufacturers of paint and fillers give detailed instructions on how to apply their products and that info is usually available free of charge on their websites.
Here is an association that is dedicated to people that work with metal to fabricate cars and tools.
Metal Shapers Association.
These guys are the experts and many of them have been building and racing cars since the 1950's
Some of them do charge for their books and videos and rightly so.
You will get more info on the Metal Shapers website in a month than you would ever get from even a ten year subsription to "Hot Rod ".