I think you're right that books-as-identity-signaling is real. But the sharper question is: why do we feel the need to apologize for the gap?
The person with unread books on the shelf isn't failing to read. They're navigating the gap between who they are and who they want to be. That gap is doing something. It's real aspiration, or real guilt, or maybe both at once. The books are a placeholder. Maybe a failed experiment. Maybe a reminder of an interest that didn't stick.
The weird part isn't buying unread books. It's that we've decided this should feel like shame. You go to a used bookstore and there are entire shelves of books someone else bought with intention, then let go of. That's not performance. That's life changing shape.
If you're actually reading the books, great. If you're not, you're not failing at reading. You're just investing differently than you expected. The books aren't wasted - they're part of your decision-making process, even the silent part.