The Marketability of Your Subject
Certain subjects are always marketable. If you venture into nonfiction writing for a living or as a side gig, you will inevitably write on subjects that are always marketable. If you become a refined or talented nonfiction writer, then you will develop the ability to write on less popular, but in demand subjects. This will make you a busier writer.
Subjects that are not well covered in the marketplace are always the best ones to write on. This is the main reason that nonfiction writers who know how to write well in the technology fields are always in demand. Technology changes constantly and if you can write well in a way to help average people understand the newest technology, then people will seek you out for their projects.
In terms of picking a subject to write your first nonfiction book on, you will want to look for subjects that are not well covered in proportion to the public's interest in them. Self-publishing is a great example. As a publisher and editor, I encounter this daily. Our company's website has a good deal of information on it that helps to educate our potential clients before they call or email us. Why do we have to post lengthy segments in our pages when it flies in the face of Internet marketing sense? Because self-publishing is a popular subject that is not well covered in the marketplace where people are looking, the Internet. There are an abundance of books on it, but not so many articles.
Of course, the other reason we have to post so much material in our site is that self-publishing is and can be a very specialized subject. Rarely will you find any two publishing houses handling it in the same manner unless they are owned by the same company. What does this mean to you, the new nonfiction writer? It means that you may be able to directly market your nonfiction writing skills or articles on self-publishing to a publishing house that specializes in self-publishing.
Remember, the editor you submit your book or article to will look at the marketability of the subject. How broadly will the book hit public interest? How specialized and in demand is the subject? How specialized and small is the market? Does the writer bring a fresh and unique treatment to their subject? Do they touch on a concern that has been overlooked or ignored? Make your treatment of the subject one that increases your book's marketability.
Core Buyers
Your editor will either need to be able to immediately discern who your core buyers are or she or he will need you to spell it out in an almost statistical manner in your submission letter. An editor wants to know, how large is this market? What is the target group? How well do they purchase books on this subject? Do they read books at all? What is the median income of the core buyers?
Remember, just because you think a target market or group is your core buyers, doesn't mean that group actually IS your core buying market. For example, if you write a nonfiction book on relationships that is targeted to young men and is based on your personal experience with young men, you may be rejected by your editor. Why? A relatively small proportion of your target market actually makes up your core buying group.
To successfully persuade an editor of such a book, you may need to rewrite it so that it speaks to women about relationships with young men. Women are the largest buyers of books on relationships and are of greater interest to an editor. Remember, your editor wants to handle your book because they know it will sell; not because they think you're a cool person (even though you may be).