If you determine that there is some demand for your product, you will need to determine how large the market is. The size of your market here, refers to the niche you are going after. It is not enough to use the size of the whole book market, when you are just targeting a part of that market. You need to know if your market is a high growth market or a mature market. You also need to know if it is a fragmented, or consolidated market.
Investors are interested in high growth, and new markets.
This is a question that has been ignored for a while in the Internet space. So long as you had a website that could attract eyeballs quickly, investors did not really care about revenues and profits. The marketplace is changing now and investors are seeking profits. You will therefore need to figure out how you plan to make money off your idea. If it is something you plan on working in full-time, you need to figure out if you will make enough money to make it worth your while.