1. You cannot write for everyone.
Don't even attempt it.
2. It is very likely that your book is not the first on your subject.
How is it different? Who would be interested in that difference?
Your target audience is defined by (a) who you are and (b) the market situation.
Who are you?
- What interests you?
- What is your level of expertise? (Can you dumb down your knowledge?)
- What is your writing style? (How flexible can you be?)
- What kind of people do you want to interact with (face to face, social media, ...)?
What is the market situation?
- What other books are there on your topic? What do they cover? What is missing from them?
- What are current trends in your field? The social and political situation? Urgent questions waiting for answers?
- Which part of the public is interested in which aspect of your field? Which part needs information on what?
- What is the buying behavior of different reader groups? Are other interests they have somehow related or relatable to your field?
3. How to
Take a huge sheet of paper, or a big pile of little sheets, and start writing down everything that you can think of that defines both (a) you and your work and (b) the market as it relates to you and your work.
Don't hurry this. Take a few days. Take walks. Visit book stores. Browse relevant blogs. Try to find resources on market analysis – not how to do it, but actual research on "what teens do on the web today" or "how elder people spend their retirement" or whatever. A lot of these studies are not publicly available for free, but some have been summarized in the media.
Don't get lost on the web or confused by the ocean of opinion. Use the information you can find not to tell you what you should do, but to understand who you are not or what resonates with you. Always come back to taking lonely walks and thinking without outside interference. Do not attempt to bend yourself to where you think an audience might be, but stay true to yourself and your vision.
Try to see trends in your note taking. Define a core focus that is narrow and clear. Try to phrase that focus in one sentence (that's your unique selling point). Define a target audience for that focus. Make this definition narrow and clear, e.g. "24 year old male sports enthusiasts working in the media and living in big cities".
- for this target audience and
- about your core focus.
This narrow focus gives your book a clear profile or "character" and makes it attractive for readers outside your target audience, too.
Note: You can always write another book on another aspect of your topic for another audience.
A book about government and decision-making? How utterly boring! I don't even vote.
But The Drug Taking Wastrel's Book About Government and Decision Making? Or International Politics for Busy House Wives? Ha. Sounds funny. Let's google that.