You've identified your target audience. Good for you! This group of readers is most likely to buy your book. Now it's time to think outside the box by choosing your secondary audience. This group of readers will likely buy your book. They will have different reasons for buying it than your target audience. Defining your secondary audience will help you make promotional decisions in the future.
Your secondary audience will be piggy-backed onto your primary audience. A connection will exists between them. Perhaps you have written a love story about vampires. The love story is central to your book, and romance readers are your primary market. Vampire fans are your secondary market. Some will be romance readers. Others won't. The vampire fan can differ greatly from the romance reader or be similar. Vampire fans might read different blogs that romance readers. They might join different clubs. The demographics are overlapped but not wholly. You will focus your efforts on your primary market, but you may consider shuttling some of your marketing dollars toward your secondary market.
A well chosen secondary market is large enough to make your promotion efforts profitable. Hence your secondary market must be broad in scope. Perhaps you write cozy mysteries all set in the west. Your primary audience is cozy mystery fans, but you are wondering about your secondary market. It may be true that all your cozy mysteries include plenty of information about an obscure town in New Mexico, however choosing the citizens of this town as a secondary market won't generate many sales. If you chose a secondary market that consists of lovers of western fiction, you are more likely to find success. A good secondary market must reach a sizable number of people. Keep this in mind as your make audience decisions.
Take time to identify your secondary market to maximize your sales.
A great book always starts off with a love story. I believe my secondary audience for "Memoir of a Sky Warrior would be for those who interest in romance memoirs.