Hunting has two main modes: searching and chasing. With searching you look for something to chase. With chasing, in contrast, you have a focus of attention that drives your actions. You may find something else worth chasing along the way, and then switch your focus to a new chase, but you’ll still maintain a focus.
It seems to me that while reading non-fiction, most folks are in searching mode. Most would be more intellectually productive, however, in chasing mode. It helps to have in mind a question, puzzle, or problem, and then read in order to answer your question, explain your puzzle, or solve your problem.
In searching mode, readers tend to be less critical. If a source came recommended, they tend to keep reading along even if they aren’t quite sure what the point is. Since authors tend to be more prestigious than readers, readers tend to feel reluctant to question or judge what they’ve read. They are more likely to talk about whether they enjoyed the read, than whether the author’s argument works.
In chasing mode, readers are naturally more critical. When you are looking for something particular, it feels less presumptuous to stop reading when your source comes to seem irrelevant. After all, the source might be good for some other purpose, even if not for your purpose.
In chasing mode, you continually ask yourself whether what you are reading is relevant for your quest, or whether the author actually has anything new or interesting to say. You flip around seeking sections that might be more relevant, and you might even look up the references for an especially relevant section.
Also, search-readers often don’t have a good mental place to put each thing they learn. In which case they don’t end up learning much. Chasers, in contrast, always have specific mental places they are trying to fill with what they read, so they better integrate new things they learn with old things they know.
In chasing mode, readers also tend to better interleave reading and thinking. People often hope that search-mode reading will inspire them to new thoughts, and are disappointed to find that it doesn’t. Chase-mode reading, in contrast, requires constant thinking, in order to evaluate how the current source addresses your chosen focus. This tends to make it easier to notice missing holes in the literature, where your new idea can be placed.
So if you read to be intellectually productive, rather than just to fill your time, consider reading while chasing something, anything. (From a conversation with Heather Macsorley.)
Added 8p: Katja and Andy comment, and dloye offers this quote from Samuel Johnson:
What we read with inclination makes a much stronger impression. If we read without inclination, half the mind is employed in fixing the attention; so there is but one half to be employed on what we read.