Usually, the way it goes, you write a book, try to get it published, and your writing and your publicist do the best job possible to make it popular. However, that might not be the most profitable way of going about it, according to Judy Collins, who gives tips on how to go about writing something that will actually sell. (OK, technically this has been targeted to self-publishers but I still think it's amusing to think, "What should I write? Well, non-fiction sells better, so I'll do that.")