I think yk's idea is excellent - I'm actually working on it right now. You're correct that the book locations need to be about as long as the book to provide sufficient unique values - but it isn't humanly or temporally possible for people to bookmark that many pages - so a separate index of bookmarked pages could use much shorter urls.

and please make the bookmarks unguessable and private. I want to sent moderately private letters, that where already "written" in the library :)

I'm glad you suggested that - will do.

letters where sent :)

I'm still amazed at the project, thank you

Not if you let the first linked page be site.com/1, the next one site.com/2 and so on, I think.

Yes, even then.

Sure "1" and "2" are pretty short. And even "57834573495879436129386943" is pretty short.

But the average link would have so many digits, I couldn't post it in this comment.

The bookmarks would only need to map to the book locations - you would only need sufficient values for the number of bookmarked pages, not the entire range of possibilities.

How does that even make sense? Taking a lower base will only require more characters.