[ndriscoll]

Hashes are not excessively long unless you choose to make it so. They might be opaque/random if you want, or they might not. "Remove all special characters and keep only the first 5 characters with space padding" is a string hash function. "Keep only the first 5 vowels with space padding" is a string hash function.

Here's a friendly AI generated hash function to give you an opaque 13 digit number if you're into that:

echo -n "$URL" | sha1sum | awk '{print $1}' | xxd -r -p | od -An -t u8 | tr -d ' \n' | cut -c1-13

For example, for https://standardebooks.org/ebooks/denis-diderot/the-indiscre... you get the ID 4897562473051.

It looks like their ebook sources are all published in git repos online, so you could check out the repos, get the timestamp of the initial commits, and do a monotonic ID on that if you wanted. You could also contribute the change back to them if you think it's something others would benefit from.