Someone recently photographed some pages using various different spectrums and found old notes from prior owners in the margins. It also showed that this wasn’t vellum that had been scraped and washed for reuse.
I just bought a book of this as a birthday present for my wife thanks to this post! (she already owns the Codex Seraphinianus, so it's a sure bet)
Every time I read about this, I just imagine some creative person with the right resources decided to dip into the fantasy genre and do world-building. I'd do similar things as a kid. I made up all kinds of fictional things and made little encyclopedias for them. It is great fun
