“A 4GB ebook reader filled with 3,500 ebooks weighs a billionth of a billionth of a gram more than if it were empty of data – a difference that is approximately the same weight as a molecule of DNA. The same number of physical books would weigh about two tons.” -From crazyfacts.com/tag/books

Wild!  For more mind blowing facts about books, go to Crazyfacts.