I think one good test is actually the table of contents. Some older books which were content-rich had the feature of "descriptive" TOCs. They would not only ha

I think one good test is actually the table of contents. Some older books which were content-rich had the feature of "descriptive" TOCs. They would not only have a clear headline, but also a sort of teaser description of what that chapter would tell you. So you could use that to get a high level understanding of what the book is about and what it would tell or teach you without even having to skim the actual content. On the other side of the spectrum, many of those sparse business/popsci/etc books often have extremely cryptic TOCs. That's a good sign to just ignore that book.