Before you can think about promoting your book, you have to identify your target market - the people most likely to buy your book. Why? Because if you don't properly target your audience, you'll waste time and money on ineffective marketing or promotional tactics. Planning is as important in marketing a book as it is in marketing a business.
On top of needing to know your market to work on book publicity efforts, you'll also need to be able to demonstrate that a strong enough market for your book exists before it's even published, if you plan to pitch it publishers.
So how do you determine who your target market is for your book?
Look to Similar Titles
Chances are that there are at least somewhat similar books already released in your niche (for non-fiction books) or genre / sub-genre (for fiction). If there are no other books available even remotely similar to your idea, it may mean there isn't a market there for the type of book you want to write. This is one of the reasons researching your competition is so important. See who's buying them. See who's reviewing them. See what people have said about them. These same people will very likely be a part of your own target market.
Perhaps you plan to write a book on parenting preschoolers. You may think "Great! There are lots of parents with preschoolers, so there's a huge market!" By targeting too broadly, you run the risk of joining a larger market that's already too saturated. You also put a limitation on yourself - a broad subject can rarely solve a specific problem for someone, and addressing a problem and offering a solution is a key element of effective marketing. Instead perhaps you might narrow the focus of your audience, and therefore your book, to a more specific niche: examples of narrower niches would include potty training your toddler, getting preschoolers to develop healthy eating habits early, or a book targeting work-at-home moms with young children that offers suggestions on keeping kids occupied and entertained while they work, without feeling like they're neglecting them. Want to know what people are interested in within a broader niche to help you narrow down your audience? Try using the Adwords Sandbox tool to see what people are searching for.
Look Beyond the Reader
It's easy to get so caught up in your end readers that you lose focus of other markets that may be interested in buying your books. Might it be a book that schools would want to buy in bulk? Would the book make an ideal gift? (For example, a book on easy cooking while you're in college would probably appeal to parents of college students as much as, if not more than, the students themselves.) Does the reader have the buying power? (Similar to the last example, think of test prep books for high school students. Appealing to the students themselves is fine, and they could be one market. But a potentially larger market would again be parents, as they're likely the ones paying for the books. Along with buying power comes influence. Maybe someone like a teacher will have influence over what the parents by, so that would be another market you would have to appeal to.)
Beyond the Obvious
If you've been reading All Book Marketing recently, you know I'm in the middle of outlining a novel - in my case a sort of modern-day fantasy horror in a generally popular sub-genre. Obviously, I would target readers who enjoy the sub-genre I'm writing in. However, to set the book apart, I'm almost completely tossing conventional elements to the wind, avoiding any kind of stereotypes. While it may make it different, it may also alienate that obvious market for the book, who might be expecting something else. So I have to think of other markets it might appeal to. It's set up in a way that it could also be classified as a psychological drama, so I know I can market it to audiences who enjoy those titles. I also don't know if it's something I want to pitch to publishers, or simply self-publish. If I self-publish, given my background in online PR and Internet marketing, I'm confident I could carve out a niche of online readers using innovative Web-based techniques that would appeal to other writers, other PR professionals, as well as the target readers themselves to draw more attention to it.
So what else can you do when trying to determine your book's target market? Can you create markets for your book? Can you look past your initial reaction, and think of other audiences that might find your book useful or interesting in some way (even if not the way you originally intended)?