We will include mistakes, not because I don’t know the answer, but because this is the best way for you to learn. Including mistakes makes it impossible for you to read passively: you must instead engage with the material, because you can never be sure of the veracity of what you’re reading.
I. As another comment mentions, this is for technical books, and seemingly specifically textbooks (AKA lessons, as opposed to the other three quadrants of technical documentation, references, instructions, and tutorials). In case anyone here is considering writing a fiction book, please do not ever expect to make any money at all. The odds are downright absurd these days.
II. I absolutely agree that it's a sometimes-underappreciated and potentially very lucrative career move if you have real expertise to share and are a good writer. Especially in the vaguely post-blog era.
III. Writing a book but not expecting to make any money from it is... bold. I'm very glad it worked for this person, and if you're looking to use it as a credential to land contracts or appointments, then that's very smart and kind. But I think I speak for many authors when I say that writing a book is very time consuming, and you need to eat somehow. Not all of us are just publishing cleaned-up lecture notes, after all.
I initially shared this person's "offer a print copy as a tip jar" concept, but have gradually moved towards "only publish free snippets" as I consider long term financial feasibility. This applies even more so if, like me, you're writing a prose book that has a 1% chance to be truly popular among laypeople, rather than more realistic textbook-centric goals.
IV. PoD is indeed incredible. For those not in the know, this allows you to sell your book without any significant upfront investment, and have them manage shipping. There's definitely room for profit via many sites; Lulu, for example, will print a 250 page "Digest" (normal) paperback in B&W for $7.56/ea.
I'm also planning on going with http://lulu.com . Arguably it's <50% likely to beat Amazon's uber-popular PoD service on overall sales+profits, but it has some notable advantages:
1. If you're thinking about a publishing a "premium" book, which some textbooks might be, they have some gorgeous options -- both in terms of color printing and cover material. https://www.lulu.com/pricing
2. If you're a web-dev comfortable setting up a Shopify portal on your own site, you can hook it up to Lulu and get "100%" (after PoD costs) of your profit. Obviously, this is potentially a huge deal -- the trick is of course getting Amazon-like numbers of eyeballs on your own site. https://www.lulu.com/sell/sell-on-your-site You can even order copies via API, which seems goofy but potentially fun.
3. They're targeted at DIY authors, and as such offer a nice little knowledge base on formatting and such: https://www.lulu.com/publishing-toolkit I haven't printed any yet, but they're templates are nice to work with.
4. They're not only "not Amazon", they're a B-Corp. Which, hey, we all gotta do what we can when we can, no matter how small.
...I swear I'm just a biased fanboy, not a shill!
V. The cover art space is fucking wild these days, AI has absolutely upended it. Scroll through /r/writers for lots of horror stories. I would be cautious about hiring any freelancers to help you without serious vetting. Plus, Inkscape is free and technical books don't need realistic art!
P.S. Does the author know they're invoking a antisemitic trope with the "(((Parenthetically Speaking)))" title? It seems to be in obvious good faith, but that did draw my attention for a short moment. Maybe just one set of parentheses could do the trick ;)