Who is your audience?
In business, there is a thing called the 80/20 Rule (originally Pareto's Principle) which states that eighty percent of your results will come from twenty percent of your efforts (contacts, sales, promotions, etc.), while only twenty percent of your results will come from the other eighty percent of your efforts. The 80/20 rule applies just as well to publishing as it does business and other aspects of life. Pay attention to the people and things that comprise the eighty percent group.
Begin by listing each identifiable group of readers that comes to mind. Your list may be short or long initially, depending on the topic of your book and the areas of interest to which it is related. Be sure to include those to whom you will send sample copies, such as editors of topically related magazines, libraries, newspaper columnists, business associates, and so forth. Don’t worry if you feel you are missing some groups or cannot think of all the possibilities. Save this list, as you will be able to add to it from time to time as new interest groups occur to you.
When you have some satisfactory number of potential reader groups, begin to sort them into two classifications: those with a high natural interest and those with less of an interest. If you have a sufficient sampling of group names, you may begin to see the 80/20 rule working even now.