On one hand it's nice to have a physical copy since you never know what will happen to your digital device and the cloud, and also a study showed we retain information better when we read from a physical copy...on the other hand I think a lot of people (like me) keep tons of books they'll never read again and it's a waste of resources.

I'd have to look through my whole collection to remember what all my favorites are, but one good one I saw hanging out on my floor is The Examined Life: How We Lose and Find Ourselves. Other than that the ones that come to mind first are all the books in the dialogue series by Neale Donald Walsch.