Hi Heidi,
What you are describing is finding your target audience, and it is important no matter if you're selling books or bangles. Publicists and others in sales do this on a daily basis. If this step is not done, your marketing efforts are completely aimed at random with the hope of hitting something/anything that counts, and that takes way too much energy.
In general it's much easier to do this for a non-fiction topic because there's basically a built in idea as to who needs to learn what you're trying to teach to at least kick things off. For fiction, it's a little tougher but not impossible.
Either way, start with men/women/kids, who like reading/learning about x and try to continue to dig deeper.
If you'd like to post a little more about your topic, we can probably help you get some ideas going.