Make a List of Things You Love
A simple thing you can do is take out a sheet of paper and start writing down things you love. Write your hobbies, your interests, your TV shows. Any category you feel like. Explore your mind and write down stuff you truly love whether it is Coca Cola collecting, bird watching, sewing or fixing up cars. When you get several things written down, sort through them and try to narrow and shorten the list down to your top 5 to 10 favorite things.
Now you will have a better idea of areas to further explore to find nonfiction book ideas. I saw a book not long ago titled The World of Cheese. To some people cheese is cheese, but to others, perhaps it is a passion or something they are simply nuts about. There are a lot of people out there, so chances are if there is one person out there crazy about cheese, there are many others that will share that same passion and will buy your book on cheese!