The simple act of asking can make a world of difference in your life. We all have friends and relatives, so why not ask them what you should write about? Ask business associates and colleagues. Perhaps they can shed some light on what to write about. What kind of book would they like to have? Librarians and booksellers are asked questions from their customers all of the time. Why not ask them what customers are seeking.
Ask those in a market you are considering writing for what they need to know. Find ways to communicate with several people within a market to get some good ideas and look for popular, recurring themes.