Every bookshelf tells a story. You can’t build one in a day or even a week… it’s a lifetime of collected changes. On the shelf over there I see an Isaac Asimov collection I bought when I was 12, right next to a yet-to-be-published galley by a friend of mine. Each of them changed my life.
If I visit someone’s home for the first time, and there is a bookshelf, I always make a point to peruse it. You can tell a lot about a person based on what books are there.