Go to the bookshops in your area and take note of the kinds of non-fiction books that are stocked. You should particularly note the ones that are self-help or how-to books. If you can create the opportunity, ask the stock boy which ones are selling the best. Hes the one who knows what is being restocked most often. That failing, ask a floor salesperson the same question. Asking the bookstore owner or manager is the last option. They are more likely to be pushing the books that arent selling well in order to make a sale. Learn by whatever means you can which self-help or how-to books are the hottest sellers.
Another option is the online bookstores. You cant, of course, ask for information from a person but you can research the site and determine which of the non-fiction self-help or how-to books are highest in sales.
Finding out what books people are most often buying can give you some insight into what the hot market subjects are. Armed with this information, you can go about creating a niche marketing website that will have a better chance of becoming successful.