When it comes to making multiple sales, it seems like fiction writers have all the luck. They can write a series of books, creating a fan base that buys each volume as soon as it comes out. Non-fiction, on the other hand, often doesn't lend itself to a long series of volumes. Non-fiction buyers are looking to solve a problem in their lives and, with luck, you'll offer that solution in your book. They want to know about gardening in Florida, or using a slow cooker, or house training a dog. Once they've learned it, they don't have that need to go over the subject again and again.