I keep seeing postings by hopeful, new writers, e.g on Quora, who are ask questions about the presumed future earnings of a nondescript book they are working on. Many of them seem to be deluded by their own dreams of success: To write a book is a piece of cake to them and to have it published by a renowned publisher doesn't look difficult to them, although they may have some vague misgivings about the well-known bad experiences of other authors. As it is, right now the publishing market seems to be swarming with fake agents, fake editors, fake publishers, etc., etc..
Some years ago I made the decision to accept the offer from a publisher whom I had been told was very shady: I accepted "A dance with the Devil" so to speak because I wasn't very hooked on the idea that i should make a lot of money on my work. To me it was a question of publishing books I myself felt for, and I chose to close my eye to the fact that this publisher more or less kept his business running not on sales of the books he published, but on what he could get in through grants from firms, etc. which wanted to boost their names by "doing something for culture". As they saying goes: "One hand washes the other one ...." However, i took for granted that this particular publisher would do what he could to market the books he published. After all, he had to get to his pockets to get books on the market, as they don't edit and print themselves. I was surprised to find out that that wasn't the case, or that it was much less than it takes to market books. As marketing and publicity (PR-campaigns) are two completely different things my experience with this particular publisher taught me that one shouldn't chose the one over the other, no, one should always go for both with every book one publishes.
Marketing may include something like e.g. digital campaigns, paid ads in newspapers and magazines, but also influencer efforts (if that's possible). As to publicity then we are entering the realm of events, articles and reviews: The more the better. As every published author knows by now it's very hard to get any of this coverage, but part of it is considered the obligation of the publisher who should be able to draw on his/her market contacts. However, what if one is working with someone who doesn't have a publicity budget and who thinks that the author him-/herself should take the load of that part of the publishing, preferably without any "fuss"? It's not fair to the author, but I think that's what many experience these years.
Some authors resort to the strategy of hireing their own publicist, but how does one know the good ones from the bad - and even destructive - ones? That's part of the problem: One doesn't because the publishing market is also swarming with people who either are fake or just not good enough. Bad advice about this part of the publishing process may have grave implications for the hopeful author and one again i'm thinking of those writers who take for granted that they may make a living by writing ....
https://blog.reedsy.com/book-promotion-services/
https://savory-pr.com/why-you-should-hire-a-book-publicist-for-your-book-launch/