Reading and writing are invariably more bodily in nature than we are inclined to think. Where we sit, what the lighting is, how much we've had to eat or drink: these physiological factors play a crucial role in our experience of those activities. There's a reason why magazines like The New Yorker have been running ads for literary accessories since their inception. The person reading the magazine while suffering from a headache or exhaustion is both aware that her or his body is conspiring against the task of reading and doing her or his best to suppress those surges of unease by focusing on the text as if it were not part of the physical realm.
Today I spent a long time sorting through crates and boxes of books and moving them around in my office, the garage, and the storage space. Although the task eventually wore me out, I was initially happy to be doing it. There's a big difference between reading a piece online, where all cats are, in a sense, gray and reading it in a bound volume you can hold in your hands. I don't mean to devalue online learning, which occupies much of my time. It's just that there's knowledge conveyed in the physical experience of holding a book that can only be distilled indirectly for cyberspace.
Recently, when I was talking to a former student about to take his doctoral exams, I explained that I had prepared for my own by both reading and listening to books on tape. When I took my orals, I found that the material I had take in by ear was more readily available for conversational improvisation than the material I had only read on the page in silence.
I wonder if something similar is at work in the distinction between reading a book and reading online. I find that the absence of distinctive visuals in internet content tends to make it less clearly resolved in my mind. I often remember details from a book in a process where look and feel are inextricably bound up with the words I summon from the depths of memory. In the absence of such mental props, I struggle.