There are two different areas we could be talking about. The best way to answer for both is to use surveys, you could go all out and hire a market research company to really find it out but you can also use some shortcuts for quick daily items:

 

(1) Business: for your website, blog or newsletter.

 

You can go to Google Trends to see what is being talked about, visit http://www.google.com/trends/and do a search on your subject. You will see related articles, what is being talked about and this will give you an idea of what you can write.

 

You can edit this by region, time of year and even compare multiple keywords by separating them by commas in the search box.

 

(2) Writing a book, more research needed but some quick facts:

 

Per Harris Interactive a survey done in 2010 found that readers who read at least one book per year reported the genres they liked were:
Mystery, Thriller and Crime: 48%
Science Fiction: 26%
Literature: 24%
Romance: 21%

 

Per Paid Content website:
“Most e-books sold are fiction 58%, with literary fiction, science fiction, and romance each comprising over 20% of all e-book purchases.”

 

Another survey, reported on O’Reilly, found that the largest percentage of ebook purchases were:
Guilty pleasure fiction – 50-60%
Nonfiction – 30-40%
General and literary fiction – 20-30%