If you’ve already gotten a copy of an old, out-of-print book, why would you want to look at a digital copy? Isn’t having your own copy the perfect way to go?
It’s ideal, but it is worth noting that other copies of that same book you have may have had notations added by the person who originally owned the book–perhaps about your relatives. Your copy of the book may be missing pages that the copy used for digitization was not missing. You may have your parent’s 1940 high school year book, but perhaps they wrote something to the person who originally had the copy of that yearbook that ended up being digitized.
If you discover that there’s a digitized copy of something you already have or have already seen, consider whether it may be worth taking a look at that “other copy.”