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.
So what's a non-fiction writer to do when she wants to increase her repeat business? If she doesn't have any recurring characters to rely upon, how can she go about growing her own tribe of fans? The answer is similar to a fiction writer's solution: write multiple books and create a platform where your fans can interact with you. The difference is in the subject matter.
Become the Expert
If you're writing about a topic, the odds are good you have at least a small amount of interest in the subject. Your first step is to become the go-to expert on your topic for as many people as possible. Social media is the key for this, with whatever site you're most comfortable. Don't mention your book, simply make a series of posts, some every day, that would interest fans of your book. Your list of friends or fans will grow organically from people sharing your posts, tweets, or pins with other friends. Mention your book in a casual way no more than once every 50 posts or so.
Choose Series Topics
In the meantime, choose the titles for your next books based on what fans of the first book would like. Use a tool like the Google Adwords Keyword Tool or the Amazon search engine to find phrases your fans will search for. For instance, if you wrote about shelter dogs, a quick search on Amazon shows topics such as stories of adopted strays and taking a shelter dog home. Further searching will result in even more topics for book titles.
Involve Your Growing Tribe
Involve your fans with the search, as well. Ask them what questions and concerns they have, and join in all the conversations. The idea is to make it a friendly ongoing conversation, not posts to advertise your book. If they like you and respect your views, they'll naturally want to purchase your book without a word of promotion.
Build Interest in Coming Books
Give hints about upcoming books to entice your readers, keep interacting with the fans and increasing the size of the group, and write more and more books on the same general topic. Do this long enough and you'll not only become well-known in your area of expertise, you'll also have a built-in fan base, just like the fiction writers.