This post is intended to be conversational. I'm noticing something about myself and changing it; take it for what you will.
I read quickly; about two and a half times as fast as I read it aloud. A more useful number might be a page a minute, but that's obviously subject to massive uncertainty. I've also read a lot of books in my life- many of them were fiction, but for the last several years it's been mostly nonfiction.
I have the feeling that I should read every word of a book. That's only half-true; as far as I can tell, I often don't read every word of a paragraph. But glancing at a paragraph and guessing its meaning is a far cry from writing off a lump of text as not worth my time. I suspect that's a habit from fiction reading- every paragraph is there because it's pleasant or it advances the story, and the story is far more valuable as a whole than as a series of disconnected events. But with nonfiction reading, the value of different paragraphs is massively variable. A Conflict of Visions, for example, is a massively insightful and valuable book. But I got the feeling when I read it that there was a lot of repetition and reiteration; to me, the first two chapters (35 pages) had about 90% of the value of the book (263 pages). If I had known to just read those pages and stop, I could have gotten 90% of the value for 13% of the time, and saved myself almost four hours.
(As an aside, I suspect this is one of the reasons blogs posts can be so valuable- many ideas only need a few pages to express, and books simply cannot be 30 pages long. Either you don't publish it, or you extend it to ten times the size.)
And yet, it is hard to take advantage of this. I'm familiar with the 80/20 principle (and, with the case of A Conflict of Visions, it was more extreme at 90/13) but there's the fear that I'll miss some gem, or won't fully understand the ideas if I don't read all the pieces. And so I do read many of my nonfiction books cover to cover. But the main thing I'm cultivating is a willingness to skip, not just words in a sentence, but paragraphs, pages, or even chapters. The main test I'm using is whether or not I want to read the sentence I'm reading right now. If I feel the least bit of disinterest, I flip ahead. (If I find I missed something, I can always flip back, and at this stage I suspect I'm so ignorant of my interest that if I've noticed my disinterest, it's serious enough.) And so those books I read cover to cover (at least, didn't skip any paragraphs) are the books that were fascinating the whole way through, not just any book I put my hands on.
At the moment, I just finished Somatics (recommended by NancyLebovitz here, with a link to a free, mostly complete version) and am reading Awareness Through Movement. I've found myself skipping substantial parts of the second one, for a few reasons:
1. It's well-organized. Each few paragraphs is introduced by a header that often will tell me I want to skip that section (either because the summary is sufficient or the content isn't worth it). The atomic nature of this- every few paragraphs instead of every few pages or every chapter- really helps because it's much easier to feel comfortable rejecting 3 paragraphs than 30 pages (what if one of those pages contains the best idea of the book?).
2. The author has a focus that is frequently different from mine. Many sections stress self-image, self-education, man's relationship to society, and other things that simply aren't why I'm reading this book. When I figure out he's going to talk about that for a while, I can go ahead and rejoin him when he's talking to me again.
3. I just read a book on a similar subject, and feel more comfortable separating useful and useless, especially since I've got a narrow definition of use. I want to develop my personal awareness and control over my muscles and posture, and the subgoal is extracting information to aid that goal from this book.
Suggestion: when you read a piece of nonfiction, have a goal in mind. If that goal is "eat up time," well, read everything! If that goal is "learn about X" then you may want to do some planning. The table of contents is your friend, and one that before this I've only used to measure the length of chapters.