You can still download specific files from torrents of many files, yes. You can pick a subset of files to download. In theory, you could extend torrent clients

You can still download specific files from torrents of many files, yes. You can pick a subset of files to download. In theory, you could extend torrent clients to support archiving some subset of the files. You just tell it to keep around 100 GB of files in a 10 TB torrent. Since the client knows the status of each chunk in the swarm, it can make smart decisions about which chunks to download and seed. Clients already let to set preferences to seed the "least-seeded" chunks. So this isn't so far fetched. The big problem with this is that it lets you work only with a fixed archive. Torrent files can't be mutated after they are created. So you'd be able to get people to archive a fixed version of a shadow library, but not an evolving one. In practice, this should be fine enough. If we had high assurance that all the books added to the library before 2022 (or something) were copied in 50 machines, that's quite useful. Being able to layer deltas on top would be amazing - you could evolve the collection as new books are added.