Kathy Sierra explains what it takes to write a non-fiction bestseller. She says:
The main problem is this: the more focused you are on demonstrating how smart YOU are, the less likely you are to help the READER feel/become smart. And that's a formula for hurting sales. The more you make the book about how much you know, the less you have something the reader can benefit from. We write our books from virtually the opposite perspective--we don't care what they think about us. All we care about is that they have an "I Rule!" experience, and that can come only from them truly learning and understanding in a meaningful, efficient, and enjoyable way. We care about their life, more than we care about anything else. And that is the formula.