As I’m about to write a technical book (my first was pretty bad, but sold reasonably well), I found “Write useful books” by Rob Fitzpatrick which approaches writing a “useful” book (i.e., something that really teaches you something) as product design, by basically product-designing your book through multiple beta-reader iterations, really drilling it down on making it useful for the readers. Part of that is cutting down on all the things that impede making progress from applicable piece of knowledge to applicable piece of knowledge such as preface, introduction, long theoretical underpinnings, etc…

I think that most textbooks ought to be “useful” books first, rather than a mix of reference material and pedagogy, tediously plodding through things in a sequence that doesn’t make didactic sense.