Telling a legitimate network marketing company from a pyramid scheme is the question that brings most people to a blog like this, and Robert Strong takes it on directly, setting out what a compensation plan is supposed to do and where the line actually falls.
He reviews companies with that question in hand, on fitness, nutrition, collectible coins and telecoms, and the reviews are more useful for asking what is being sold than for reaching a verdict.
The traffic material is technical in a way the rest is not. What a backlink is and how to get them without buying trouble. What anchor text does. How forums still send people if you are not obviously there to sell.
He recommends the books that shaped the industry and reviews one very old one about the power of thought.
If somebody has invited you into something, read the scams post first, then the review of that company if he wrote one.