This is written by Matthew Butterick—a lawyer, typographer, and programmer. He's got another online book called Practical Typography that will help you appreciate (and make!) good typography: https://practicaltypography.com/

The site is really fun: at the bottom you can change the body text from Valkyrie to Equity, Concourse, etc. (these are all fonts that he made).

His books are made with a Racket-based publishing system called Pollen. I've used it a little bit and it's nice: it's incredibly flexible, so you have to do a lot of work to get what you want out of it, but it also doesn't confine you.

He's made some gorgeous typefaces: https://mbtype.com/ His license is far and away the most permissive non-OFL license I've encountered: buy the font once for the lowest price I've seen in a professional font, and then you can use it pretty much everywhere indefinitely. So nice.

I use two of his typefaces (Valkyrie, similar to Palatino, and Hermes Maia, a sans-serif based off of a German typeface) on my blog so you can see it in action: https://lambdaland.org/