The Voynich Manuscript is the world’s
most secretive book. It’s dated to the
15th century and contains an unknown
writing system that even renowned
cryptographers have never deciphered.
Illustrations in the book suggest it may
cover topics like herbal remedies,
biology, astronomy, and the zodiac, but
no one knows for sure what it really is. Source

It’s obvious, isn’t it? I think it’s just the first Dungeons and Dragons Dungeon-master’s handbook. Although, it might be interesting to learn a bit about it.