Ask the Experts
Ask experts to help you find book ideas. Ask librarians and book buyers at specialist book stores. Ask if they know in which fields new books might be needed. Do you know of experts that might live near you or someone you are interested in interviewing?
Perhaps they can give you more advice on what topics to write about and what topics not to write about. If you have already decided on your topic, interview experts to get more specialized information you might not find by researching.