If you don't know the already-published books in your genre, find out. Now. That's part of what agents and editors expect you to know. And this applies to fiction as well. Good fiction carries a theme—recovery after the loss of a loved one, forgiving someone who has wronged another, or solving a particular crime.
Not only do you need to know who would buy your book, but that group needs to be sufficiently large to merit a company to publish your book.
In 1988, when I first spoke with an editor about a book for people with dementia, he said the audience was too small. I disagreed. I wasn't writing for those with memory impairment, but for their families and friends. I pointed out from my research that 5.5 million people every year are diagnosed with Alzheimer's and my estimated audience would be four times that number. Every year, more than 5 million new people face dementia, and that circle grows larger and larger.
That publisher passed on the book, but another company bought When Someone You Love No Longer Remembers and the book has become one of those evergreen books. That is, it doesn't have huge annual sales, but it sells a few thousand every year. That book has been in print for four years. It will probably stay on the publisher's backlist for a decade.
I persisted with selling that book because I knew my audience and believed it would attract a large enough group to merit the publication.
1. What are the other titles on this topic?
2. Is my target audience large enough to merit a publisher's investment?