Authors have limited time and almost no budget, and the advice available is overwhelmingly generic. So here is a blunt sort of the activities that reliably move books from the ones that mostly consume the afternoon.
What works
Writing the next book
Unglamorous and true. A backlist is the most effective marketing an author has: each new release brings readers who then buy everything else. Series fiction compounds this, because a reader who finishes book one has an obvious next purchase. If you have to choose between a month of promotion and a month of drafting, draft.
The cover and the blurb
These do more selling than everything else combined, and they are largely within your control even with a publisher (you can at least raise concerns). A cover has one job: to tell a browsing reader, in under a second, what genre this is and what mood to expect. A cover that is beautiful but signals the wrong genre is a commercial failure however good it looks.
The blurb is the second filter. Short paragraphs, a clear central problem, and a stop at the point of no return. If your blurb needs three characters' names in the first sentence, rewrite it.
Your email list
See building a newsletter that readers actually open. The only audience you own outright.
Being findable
A reader who hears your name should be able to find, in one search: what you write, in what order, and where to buy it. A single clear website does this. Astonishing numbers of authors fail it — no list of books in series order, no buy links, a bio and nothing else.
Reviews, from readers
Reviews affect both browsing readers and retailer visibility. You cannot buy them and should not try. You can ask: one line at the end of your book, and an occasional ask in your newsletter. Asking directly is by far the most effective and most underused thing on this list.
Librarians and independent booksellers
Chronically underrated. Both hand-sell constantly, both are genuinely delighted to hear from local authors, and both have long memories. A polite email to your local library offering to talk to their reading group costs nothing and can seed years of borrowing.
What mostly does not
Posting about your book on social media
Your followers are largely other writers, and they are not your market. Reach on promotional posts is throttled almost everywhere. Social media can work — but as a place to be interesting about a subject adjacent to your book, slowly, over years. As a sales channel it disappoints nearly everyone.
Blog tours and paid "features"
Sites that charge to feature your book are usually read by other authors who paid to be featured. Check who the audience actually is before paying.
Giveaways of the book itself, ungated
A free book given to someone with no interest in the genre produces no review, no word of mouth, and occasionally a bad rating. Gate giveaways behind something that indicates genuine interest — a newsletter sign-up, a genre-specific group.
Bookmarks, postcards and swag
Pleasant, and there is no evidence they sell books. If they make signings more enjoyable, that is a fine reason. It is not a marketing budget.
Chasing every new platform
Being early somewhere occasionally pays enormously. Usually it costs six months. Unless you enjoy the platform for itself, let others test it.
The honest summary
Most books sell steadily rather than suddenly, and the compounding factors are: more books, in a series, with covers that signal their genre correctly, findable by a reader who has heard your name, with a mailing list you have been building patiently. There is no single campaign. There is a backlist and a list of readers, both of which get bigger every year you keep going.