How to read a book

Here's what William Sprague has to say on the subject -

All your reading should be, as far as possible, with some definite object, other than merely to occupy your time. If you have no object in view, you may be sure that you will accomplish none ; and thus your reading will be at best a mere waste of time, and not improbably, will be fraught with positive intellectual or moral evil. When you take up a book, decide if you can, from its title, or its table of contents, what good purpose you can accomplish by reading it; what faculties of your mind it will be likely to improve; or what moral dispositions to refine or elevate ; and having settled this point, if the book be worthy of your attention, you can hardly fail to be benefited by reading it.

Another remark, closely connected with the preceding, is, that you should never allow yourself to read without reflection. There is no habit more easily acquired than that of occupying the eye merely upon an author, and leaving the mind to its own wanderings; and there is scarcely any habit, which in the end, more completely unstrings the intellect, and renders it incapable of commanding its own powers. The legitimate design of reading is, not to supersede, but to assist reflection ; not to put the faculties to sleep, but to brighten them by active exercise. Different books, it is acknowledged, require different degrees of mental exertion ; but you may take it for granted, that a book which is not worth the labor of some thought, is not worth the labor of reading. Whatever. book you may have in hand, let your mind be just as intensely employed as is necessary to enable you to realize the full advantage of reading it; that is, to enable you to comprehend its full meaning, and to give it, so far as may be desirable or practicable, a lodgment in your memory. If you find your thoughts, at any time, wandering obstinately from your author, and if no effort will bring them under your control, so that you can read to advantage, (and such cases will sometimes occur from mere physical derangement,) better lay aside, your book than to continue reading in this attitude of mental vacancy. You will be none the wiser for what you read, and you may be forming an intellectual habit which will diminish your power of acquiring wisdom in more favored circumstances.
(Letters to a Daughter on Practical Subjects, pp. 61-63)