It's OpenTimestamps.

OpenTimestamps: a timestamping proof standard

OpenTimestamps aims to be a standard format for blockchain timestamping.

The created_at timestamps that are included in Nostr events are self-declared and can be faked. Instead of relying on these, Nostr events can get objective timestamps that are recorded on Bitcoin, proving that the event must have existed at the time of some Bitcoin block. To do this correctly, it's necessary to timestamp a concatenation of the event_id and the event's signature. These timestamps can be created efficiently for large numbers of events using Merkle trees, and it's pretty quick and easy for clients to verify that an event has a valid Bitcoin timestamp. For Project Gutenberg, timestamping these books on Bitcoin would prove that they existed in the exact form in which you recorded them as of, say, June 11, 2026. This would give people the ability to confirm that these books have not been altered since that time. There have been attempts to alter classic works to fit current sensibilities, and there could conceivably be attempts to either remove the unaltered versions from the internet altogether or to obfuscate what the original version was. It's possible that someone has already timestamped the Project Gutenberg texts, I'm not sure. But what you're doing seems like an opportunity to do so.