So sometimes it is the book. (this happened because it went through like 23 editions and edition 20 was a 'modern rewrite' which killed it. The reputation and praise was gained from the older editions).
True, but not always. Just as often, such parts are terribly written for one or the reason. It's possible that particular section of text is the least favorite of the author so they rush through it. Another possibility is that the author gets tired by the time that section rolls around and just goes thru the motions. Or that the author plain doesn't think a certain section is important enough to expound on it carefully. Yet another possibility is that the author is not universally well-versed in the subject they are writing about: they know some parts better than others.
Your next task after learning 100 basic words in English should probably not be Shakespeare.
I agree that every chapter should start by connecting the material to past knowledge, and briefly reviewing some of the prerequisites, but then surely the author should get to the new stuff after that?
Perhaps your estimate of 20% doesn't include the repetition and worked examples? In general when a new concept X, I try to explain it at least three times, once informally, with terminology and definitions, with formulas, and with extra explanations. In my mind all these repetitions count as "new stuff," since they cover the material at different depth and bring new dimension to the concept.
Perhaps you could give some examples of the 20% new max sweet spot? I'd much appreciate.
I like this approach better, because most math books have some easy introductionary chapters and then it gets harder and harder, because the authors assume that they laid out the basics in the first chapters. The authors might be right technically, but it's very hard for the readers to go through a chapter that is 100% new material.
In "Seven Sketches in Compositionality" they try to somewhat reset the difficulty for each chapter by starting again with introduction and motivation for each chapter separately. This makes it much easier for me to get into the material of each chapter and to read the whole book.
People have spent decades trying to solve difficult problems in mathematics and science. A big reason they didn't get frustrated and give up after 4 minutes was largely because they didn't expect it to be a 2 minute problem.
...Like the kind you might get from a misleading Who This Book Is For section?
Maybe it's school, kids are used to knowledge being drip-fed in a pedagogical fashion, which I guess is fine, but it's also far away from how learning happens outside of school. In the real world, it's normal to encounter problems that aren't trivial.