Mark's www.selfpublishingformula.com video series on Facebook ads is very valuable. Free, too. He's working on a paid course on how to make the most from Facebook ads. As an experiment, I added something like 70 subscribers to my newsletter on a budget of £20. I was giving away review copies of a novel not yet released, so hoping that will lead to reviews, plus the small spike in sales I saw more than paid for my outlay.
When I have more of a back-catalogue I will look at doing more like it, but for now - as Mark advises - I'm writing more, and writing often.