When considering publishing a book, one must consider how widely applicable the subject matter is and how widely the publisher can actually disseminate your work. I just published my fourth book, seventh if you count works that I have written for hire. I do not expect it to do as well as my previous works because the publisher is smaller and the domain of application is much more restricted. The publisher of my first few books is a subsidiary of HarperCollins and runs bookstores itself, so its channels are much more pervasive. This is shown in the royalty statements.

Self-publishing naturally boosts the ROI on small runs, but it is not without its drawbacks. Whenever I have thought of a book that I might self-publish, I question whether it is worth writing. The major publishers know their customer better than I could realistically hope to ever know. If they would not take it on, one must wonder about the market for it. I do not say this to mean one should not self-publish, but self-publishing is usually reserved for pet projects that one has developed as a hobby and want to share. If one is looking for a high ROI on the effort it takes to write a book, the larger publishers are the way to go.

Also, the longevity of one's work is directly related to how evergreen the content is. If one writes something sexy or novel, I suspect one would be lucky to get a year or two of decent sales out of it. I write as evergreen as I can. so to this day, I am still receiving cheques from a book that I published 4-5 years ago that are close to the same level as what I received originally. It remains in the top 3% on Amazon.com. This is highly unusual, but it can be done. You will seldom get rich off a single title, but you can develop several streams of passive income if you write well and can engage readers. My 2¢, fwiw.