Read this: http://craigmod.com/journal/ipad_and_books/ And then think about your own library. Craig is right on target. Which books do you really need to keep? Which ones would you like to store on hard drive that takes up the space of a wallet? I have a library of Oxford Classical Texts that I'd just as soon boil and eat as put in a box and take to the book donation bank. Throwing out my Homer would be like throwing out a picture of my wife's great-great grandmother on cross-country skis at the age of 90 back in the old country. Except the country's even older.
Back when there were real bookstores, there was a bookstore in Philadelphia called William Allen's. He was one of the top sellers of classical libraries in the country. When a serious scholar of the classics -- usually but not always a professor -- died, he would be summoned to buy the library whole. When you bought a book there, you often knew whose library it had come from, not because there was a name written in it, but because he would tell you whose it was.
Not all scholars or serious readers keep their books for a life-time, but many keep most. And then there is the pulp which comes and goes -- kindling for the kindle.
In many cases, a library follows an individual through the course of an entire life and disintegrates only at the end of life.