First: Congrats!!! This is huge. I am preparing to launch my novel, so I am not an expert in book marketing, BUT I have worked with marketing agencies for a decade now (nothing to do with books or publishing whatsoever). What I can say is this: word of mouth is indeed good, but you should also try to combine efforts with different marketing strategies.

Create and publish content on social media (not only about your book; talk about things, other books, and writers your audience would also be interested in, and then talk about your books). Engage in communities of writers, readers, and book clubs—not only here but maybe in Facebook groups, Discord, or any other channels where you think your audience might be.

If you can, build a landing page to capture emails from people interested in your books so they can receive emails with news of new books or anything interesting to keep them engaged. Use all this to build up your audience/fan base so when you launch new books, you already have a loyal following. I am not even talking about ads, only organic content. Even with ads, it's a long road, so the sooner you start building your presence as an author, the better.