I recently started buying paper books. Yes, it's not particularly ecological, but I found that I'm able to focus at the text much better this way. My Kindle (despite plenty of obvious advantages) just doesn't really work for me. It took me years to realize this, but I always start to tinker with the brightness settings, switch pages back and forth, go into the book library and back, play with highlighting words, etc.…
However, paper is still the best format by far for text books and references. I can't stand having to use a screen for that. If I'm using a reference sheet I always print it out.