You have to write a book because you believe it has helped you, because you believe it has helped others personally and you are dying to share with it others because you know it will add value to their lives. You write it for them like a gift. You don't want anything from them. You don't want them to do anything for you. You don't even care if they all share the book with their friends, they don't all have to buy them. You're just dying to share this idea with people. Your challenge is to write it in a way that is compelling, enjoyable to read so that they will get the idea.