When making new acquaintances, and visiting old friends, I am interested in discovering the titles on their shelves, how they arrange their shelves, and the construction of the shelves. I am also careful as to how I look: this is a personal area & reveals the person, so looking needs to be respectful. Some people’s books are not there to be read: they are a display, a furnishing item. Some are reference material; some food for the soul; some from a particular period in their life and their reading is now somehow frozen in time and/or at an end; some books are a statement of personal interests & affiliations, rather than material for reading; some gifts from friends & and/or remembrance of friendship; some a field of information & reference; some a connection to an author and/or the source that informed the author, a current of energy that an attentive reader can access. And some books are books on books.
In my library, I have all of these & probably a few more.