Before you write even one word of your novel or nonfiction book, your first step should be to identify your target audience. Ask yourself, "Who will read my book?" (BTW, the answer is NOT "Anyone who can fog a mirror.")
Marketing experts, literary agents and book editors all agree that the more specific you can be about your audience, the more likely that your work will be saleable. Guidelines for defining your readership include the answers to these basic questions:
What is their gender?
What is their age range?
What is their economic bracket?
What is their level of education?
Where do they live?
What do they want and need?
What do magazines/newspapers do they read?
What are their favorite TV shows and/or radio programs?
What Internet sites do they visit?
What social networking sites do they use?
Armed with this knowledge, you can target your writing by "talking" directly to your audience. For example, you would use different tone and words when writing to a senior audience than to teens; women rather than men; techno-savvy vs. computer newbies, etc. Highly targeted writing with idioms and phrases the audience wll recognize is far more effective than bland, "this has to appeal to everyone" writing.
In addition, knowing the habits and haunts of your readers enables you to find them and market your work! A previous post highlighted how one author googled three words that described his target audience, found and joined the discussion groups where his would-be readers chatted, and drove enough traffic to his website to secure a publishing contract for his novel. Other steps you might take include writing articles for the publications your audience reads, commenting on popular blogs about your topic or subject, speaking at professional associations or memberships groups where your readers are found, and so on.
A last, but not final, reason to know your readers is so you can position yourself and your work to provide new/different information, solution to a problem or entertaining material for their enjoyment. Writing that caters to the readers' interests and needs is an almost sure winner in any market.