Go to the Market First
Probably the most important advice regarding finding and choosing a nonfiction book topic is to go to the market first and find a book for that market rather than finding a topic first and trying to find a market for that topic. Keep this in mind in all areas you search for topics and when narrowing down your topics of your chosen book idea.
There are many ways to go to the market first. You can do so by looking through magazines that are targeted to a specific group of people. Mailing lists are another way. Anywhere large groups of people gather with a common interest. Find ways to go to specific markets first, whether it is through magazines, newsgroups and message forums on the Internet, mailing lists, etc. Go to the market, and then search for a subject that sells.