The marketing you largely have to do yourself, and there are a few different approaches. For business books, the main one is to treat the book as a regular old problem-solving-product and do content/permission marketing with a focus on building the mailing list.
For fiction, it seems that the most successful authors are serialising their full-length books into a number of chapters that they release monthly on Kindle Direct for $1-3 each. They focus on building their catalog of titles over time to slowly build and retain fans who then [hopefully] keep buying new titles or binge-read the whole back catalog.
There are also some premium consultancies that help self-publishing authors have a publisher-quality process through and through, for a price more like $10,000-30,000. And finally, you've got the parasites who will "offer" to "publish" your book for a small fee, which basically means the author is the customer.