With the advent of e-books, it's now possible to own the same book in both digital and print versions. I've chosen to keep some of my favorite books, such as Zen and the Art of Motorcycle Maintenance, With Strings Attached, and To Say Nothing of the Dog, in both formats. My paper copy of Z&AMM is pretty beat up by now, but it has a lot of underlining and notes that I want to keep. (I have highlights and notes in the Kindle version too, but they may not be identical.) Other paper books, particularly novels, I'm willing to sell after I've purchased a duplicate electronic copy. I haven't added new bookshelves to my office for several years, so even though I don't buy paper books as much as I used to, I still need to make room for new books. Finally, there are other books, such as The Story of Philosophy, that I have in paper but not in e-book format. I think I did download the sample at some point, but I will probably wait for another sale on the e-book before I buy it.
If you read both paper books and e-books, do you keep books in both formats, or does one format replace the other? Please feel free to discuss your format preferences in the comments.