There is no single source of knowledge. Humans learn by having a need for something and then finding ways to achieve that goal. Our knowledge of magic goes back for thousands of years with a bit added here and another bit added there.
Wicca, which is a very modern religion started in the 1940's, is the benefactor of magical knowledge accrued through the centuries from cultures all over the world. In fact, I don't know of anything in the way of magical practices that Wiccans perform that wasn't drawn from some other form of magical belief system.
You might find the book "The Western Way" by John and Caitlin Matthews to be an interesting read on the subject of the development of the Western esoteric traditions.