"Books, you know, Charles, are like lobster shells, we surround ourselves with 'em, then we grow out of 'em and leave 'em behind, as evidence of our earlier stages of development," Lord Peter Wimsey, in The Unpleasantness at the Bellona Club, by Dorothy L. Sayers.
It would be quite interesting to go through one's own book collection with this in mind. I would have many books that I haven't read at all, such as the one on ships. I just picked it up because it was free and ships interest me. I have a lot of more or less pop theology and spirituality from my earlier years as an active Catholic. I, of course, have a bunch of technical theology from my graduate school days (now I depend on libraries for my technical theology). I have a whole host of novels I've never read (or, in the case of The Brothers Karamazov, never finished, though I got to around page 600 at least 3 times).
A lot of my books are in boxes in the garage. Not enough shelves or space to display them properly, or at all.
I read a lot of children's literature these days.
I have a huge Thomas Merton collection, 'though I haven't read him for well over a decade, it seems.
A lot of books I've read I don't own any more. Like all those Asimov books from my teen years.
We do have many copies of each volume of The Lord of the Rings. That says something, although it has been a few years since I've read it. Maybe that can be my Lenten reading.