You can learn how to build a car, build a shelf, or run for president for free on the Internet. I don't know about you, but I'm the type of person that wants to have the actual manual in front of me when I build something. I'm not likely to trust random information on the Internet when I go to build a car.
For these very same reasons it makes sense to have the gold making manual in front of you when you go to make gold. The Secret Gold Guide is the gold making manual for World of Warcraft.