You must be interested in the subject. If you write about something just because you think it will be a big seller, writing will become a real chore. You should hope that you won't be just writing one ebook about this subject. At the very minimum you will be writing articles to promote it. Hopefully, you will want to write another on this same subject and another. Think greatness.

Concentrate on a subject in which you feel confident about your strength. While you still will have to do research on any subject, this process will definitely be easier if you are knowledgeable about your topic.

You have to write information that people want to buy. In fact, doing your research will help you determine what people are buying. If a person buys a book about their hobby or a special interest, they are extremely likely to buy more.

These are some resources you can use to explore what kind of information people are buying.

http://amazon.com/gp/bestsellers/books
http://magazines.com
http://50.lycos.com   (Searchs on the internet)
http://dir.yahoo.com/recreation/hobbies   (Popular hobbies)
http://pulse.ebay.com   (What is being searched on eBay)            http://pages.ebay.com/sellercentral/hotitems.pdf
          (What is being bought on eBay)
http://www2.shopping.com/top_searches   (Shopping site searches)http://buzz.yahoo.com/overall/