My book collection / audiobook collection fits on this, with tons of room to spare:
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Even a single shelf bookcase is overkill for storing it. I could probably fit a couple dozen of them in a 35mm film canister (boy, I wish you could still find those, especially the metal ones!) If a bug out is warranted, it rides along easily - as long as you don't lose the tiny thing! Try that with a house full of paper books.
But speaking to the original problem introduced in this thread: If you have a spare room or a walk-in closet that you could clear out, you could construct floor to ceiling shelving to cover every wall. And then rows of floor to ceiling racks in the center to hold more books. It may not be the most elegant looking library, but it would be compact and highly functional. Find a broken refrigerator and put that in the back corner to serve as a fire-break for your important books stored inside it. You would have to construct internal shelving to hold the weight - standard refrigerator shelves would probably collapse with only a handful of books.