Maybe this is outside of the thought experiment but I would focus on digitization. Text compresses very well and you can fit 100Gb on a CD sized disc with an estimated 50+ years lifespan (m-disc). So you could easily fit over 30 million text only books on a single 100 disc spindle which is the size of 3 small physical books. Add some redundancy and it might be 25 million books. Books with images would be slightly less compressible but you could still fit 100s of thousands on a single spindle with redundancy. Get yourself a small bar sized wine fridge to control humidity and you could probably fit every book every made in there.
This all assumes you want to preserve the content of the books and not the books themselves. You obviously can't digitize every aspect of a physical book like the ornate artwork on the spine etc. in which case I would focus my preserving efforts on those books and digitize everything else.
Well I was 5 and my friend's dad drove me home