A 240-page illustrated book in a writing system nobody has ever read.
Documented
A real 15th-century artifact, still undeciphered
Carbon dating places the vellum in the early 1400s. Statistical analysis shows the script has genuine language-like structure: word patterns, grammar-like rules, topic consistency across sections. Whatever it is, it was written fluently by someone.
Assessment: The artifact is real. Everything about its content is open.
Stories
A lost language or cipher of secret knowledge
Candidates offered over a century: a coded alchemical manual, a women's health text, the work of Roger Bacon, an unknown isolated language, Cathar heresy hidden from the Inquisition.
Assessment: None has survived peer scrutiny. Every claimed decipherment so far has collapsed.
Interesting claims
An elaborate hoax
Some researchers argue the text is meaningless, generated to look like language, perhaps by its creator to sell to Emperor Rudolf II, who paid 600 gold ducats for it.
Assessment: Possible. If true, the hoax has outperformed most real books ever written.
Sources and further reading
Beinecke Rare Book & Manuscript Library, MS 408 catalog record
Georg Baresch's 1639 letter to Athanasius Kircher
University of Arizona AMS radiocarbon dating, 2011: parchment 1404-1438
Worth asking
Is a text still a text if no mind alive can receive it, and does that make the writer more alone than anyone in history?