Estlor said:2. Consider the product mix. In the table top RPG industry, different types of products cater to different types of customers. If you boil it down to the basics, however, every book is a combination of crunch and fluff. Most campaign worlds are about 80% fluff and 20% crunch. The bulk of the rules you need are in the Core Rulebooks, you just spin new info around them. Of course, only fanatical gamers who love your setting will buy Fluff books. The more crunch you put into a product, the higher the likelihood that someone who doesn't play in 3P's setting will want the book anyway. This, of course, leads into advice #3.
Don't forget the cream. Take a look at a GURPS supplement, and you know what I mean. GURPS Space was full of crunch, but it had some cream.
Cream is what makes it more interesting.