Honestly? Paying for a popular YouTuber to review it is probably the best bang for your buck these days. But also book signing tours do still help. And conventions. And giving it away absolutely will work, particularly to people who work at book stores or libraries, basically anyone who is highly likely to have opportunities to recommend books. Giving it away at conventions is also helpful. Getting recommendations from popular authors is also huge. And of course, nothing compares to the sales you get from a TV or movie adaptation. LOL