Any subject that someone will pay me to research and write about. So far, war, the media, arms control, organic farming and a wide variety of reports that were not published under my name, but which I authored either on my own or in collaboration with someone else, and was paid for (if the pay is good, I don't mind my name not appearing!)
It helps if you can get some recognition as an expert in your field and get recommended to a publisher by someone they trust. You need really good research skills and a willingness to cut down your work ruthlessly - you may have 100,000 words of utterly fascinating material, but if your publisher wants 50,000 words, you cannot be precious about it!