Since we are on a roll with old books and manuscripts, I give you the 240-page Voynich manuscript. It is an unsolved enigma: a manuscript found in Italy; the paper has been dated to between 1404-1438. It contains text in an unknown script, unknown language, and illustrations of non-existing plants, constellations and humans apparently doing inexplicable things.
No one has been able to decipher it. This is not for lack of trying. Cryptographers, linguists, codebreakers, statisticians, computer experts in all sorts of fields have tried; professional and amateurs alike. It seems to conform roughly to european language structures, but is inconsistent. Some believe the whole thing to be nonsense, the scribbling of a mad person. Or a personal secret language. The illustrations are fantastical, and the objects depicted does not correspond to anything we know. The book seems to consist of six sections, each dealing with a subject.
Of course, the idea that the whole thing might be an elaborate hoax might well be valid. The book is not possible to decipher because there is not actual content. We might never know.