What kind of newspaper do you have?
Is it one that people subscribe to?
Or, is it one that is distributed for free?
On the former, that means people pay to receive the newspaper, which means it is more likely to be read, which increases the odds of your ad being seen. At that point, it's a matter of how well that newspaper audience aligns with your book's audience.
If the newspaper is the latter, drive around town a day or two after delivery day to see how many copies are still in driveways. That can give you an indication of how many people might actually be picking it up and reading it. I know in my community, we have a free newspaper (they ask for voluntary subscription payments) and, often, you'll see them sitting in driveways until trash day, when people finally pick them up to toss them in the bin. If that's the kind of newspaper you have, the ad is probably only going to be seen by politicians checking to see what has been said about them, business owners checking to see where their ads were placed and senior citizens checking the obituaries to keep score on whom they've outlived. If any of those are your potential audience, you might be golden.