Distributed, self-repairing storage would seem to be the way to go. Anna's Archive is huge so practically it might be necessary to prioritize the most threatened books. And someone would need to pay for the storage, whether it's in data centres or in people's homes. I remember many years ago experimenting with Freenet and Tahoe-LAFS, but I don't know if anyone's using systems like this for books, or if they have the adoption they'd need to be really robust.