You've chosen a tough nut to crack. The reason most of the advice you have found is that most self-published writers produce fiction. Why? The market for fiction is many times larger than the audience for non-fiction. If there are three readers for "Zany Zombies of Zindalia", there might only be one for the non-fiction, "Zambian Zombies". Don't go fishing for whales in a bathtub. Trying to get anything published is very difficult, and so there is now a strong trend to Self-publishing. Traditionally self-publishing involved paying a printer to set the type, print the pages, bind the whole, and deliver the copies to the person paying for the work. That had some real problems resulting in author's having to market the material. The end result was that authors sold copies to family and friends, spent time trying to convince bookstores to carry their book, entertaining book clubs, and doing silly things to draw public attention. That often leaves the author large negative profits, and a garage full of books to collect dust.
Times have changed, and I know several authors now self-publishing on the Internet. See Amazon and Kindle sites. These sites accept manuscripts without vetting them for writing, facts, or marketability. I believe they impose a uniformity of layout, type, etc., but maybe not. They then make the work available on-line without printing anything physically. See a book title that's interesting, you can download and read it for really small cost. Site statistics is a means of measuring how popular the work is with the market the site reaches. A text that is downloaded might cost the reader, say five bucks, will result in the author getting a royalty check periodically. Author's royalties depend on the number of downloads, and the number of actual printed copies made and shipped. A colleague in Texas has, I think four books in a Sci-Fi series in the Internet publishing area. I haven't talked with him lately, but the last I heard he claimed to have earned a couple of hundred dollars a year. Keep your day job.
The popularity of your book is going to depend largely on how well it is written, and in self-publishing that means you are the editor. Readers who stumble over writing in he first few pages, are going to be lost and their comments about the book will be negative. Self-published authors are too close to their own work. They love a character, or chapter, and won't cut it even though the book would be better without them. It is amazing how infrequently self-publishing wannabe's fail to use their spell and grammar checkers, or whose writing is unstructured and difficult to read.
In traditional publishing, the publisher handles the editing (often including copy editing), book design, selection of the most appropriate type face and size, how blocks of text fit onto the page, and above all the marketing. Traditional publishers are highly selective and only publish books that are likely to become profitable. They have a track-record that gives powers of persuasion over book store owners. Publishers can arrange author interviews in popular media, and book signing tours. They can find dozens of famous (profitable) authors to review the book and say nice things about it. The hire experienced and proven illustrators and designers to make dust-jackets that are eye-catching and tempt casual sales. As a self-publishing author, you have to do all those things yourself, and out of pocket on spec. For every profitable self-published books, there are thousands of profitable traditionally published books.
On-line publishing is less expensive and gains much wider exposure than self-publishing a few thousand paperback copies. Even if you've already paid to have your book printed and bound, you can still try finding a place for your baby on an Internet site.
The competition on the Internet for non-fiction isn't so fierce, because most folks who read Internet books are more interested in fiction. Your competition will be less, and the quality of the competition is also less. The reason is that authors of non-fiction with credibility in their field of endeavor are generally going to be published traditionally. here is an example, take as a title, Chester A. Arthur: bad boy who did good.
Copy A is by Dr. Fellantini, professor of History at Harvard who has published a dozen books on the last quarter of the 19th century, with eighty pages of Bibiliography, notes, indexes, etc. Copy B is by Don Dooly. Which is most likely to find buyers? Not fair you say, quite right. That's the way of the world, few things are fair.
To win the bet, you need:
1. Have a subject that will appeal to the largest possible market and audience.
2. The text must be properly edited, with no mis-spellings, or grammar faults. The sentences need to be short (14 words, average). Use of modifiers and pronouns kept to the minimum. No fancy vocabulary. Just stick to commonly understood words and phrases that any Middle-school reader can understand on sight. Modern readers do not want long and passive narratives, so stay active and move the action along quickly. When your reader pauses over a page there is a risk that they will drift away to something more interesting/entertaining. A text that isn't fast and easy to read these days, isn't going to do much business. Remember, your job as the author is to communicate "something" to a reluctant reader.
3. Know your subject better than anyone else. If you aren't quite sure who Chest Arthur was, then don't compete with someone whose spent years studying him. There are some very knowledgeable amateurs in some fields, like the American Civil War, but those tend to be fields where the competition is found in thousands of titles many of which were written by professionals. Make one tiny error, and you might as well fold your tent. Non-fiction self-published books that do well are carefully tailored to fit into smaller specialized niches. For instance, Cookbooks and "How to" books tend do well since there are good sized markets for them, and the authors are probably no better known than you are.
Writing is tough, but to make even lunch money from it is extremely difficult. Grow a thick skin, and learn to accept "failure". Focus on doing the best you can, and from practice learn to be better as a writer and as a researcher. You almost certainly will not be remembered as an author, so concentrate on being a human being sensitive to the world's suffering.
BTW, I'd like to read a selection from you beautiful baby.