It might take some work but you'll definitely be able to get advertisers. The first step would be audience surveys to find out who your readers are and what they are interested in, and then you can go out and find advertisers who want to reach those folks.
However, if you're already doing books, what about Amazon affiliates or even, depending on the topic of these books, selling related products? If someone is interested in finance, business, or home improvement, for example, there's a lot of items they might buy beyond books. You can recommend them and make some money off each sale.