Therefore, you should read a book in several short sessions of one to two hours apiece, rather than one long marathon. If you follow the method given here, you'll go through the entire book at each session. In between, your unconscious mind will process some of what you've read.
While this example might be clear for seminar reading, things get more complicated when you read for a paper or for comprehensive exams. Here, you should give yourself more time to discover the context and your goal as you read for a still-evolving purpose. This requires active reading, great notes, and frequent breaks to let your brain catch up. Plan an hour to peruse the sections of high information content in each book. These include the table of contents, index, bibliography, introduction, and the beginning and ends of each chapter.