Check Books at Specialty Stores for Ideas
You can find other books on many subjects at specialty stores that you might not otherwise find in the bigger chain book stores. You will find more specialized topics. Specialty stores cater to a highly targeted market and you can multiply your market if there are many stores in different cities throughout the world. Take notice of them in your area and when you travel.
Examples of specialty stores include hobby stores, sewing stores, greenhouses, bicycle shops, saltwater aquarium stores, magic shops, ski shops, pet stores, etc. These are stores that already have proven to have a group of people with a similar interest that buy. What book could you write about that would help them out, improve their hobby, solve their problem, etc? Are there any specialty stores that you go to, are a part of the market and know like the back of your hand? What book if it existed would you buy if you saw it in the store? Is there a book missing that should be selling? What void could be filled?