The Voynich Manuscript, which was purchased by and named after rare book dealer Wilfred Voynich in 1912 from the Roman College, bears the title of “The World’s Most Mysterious Manuscript” (Robert Brumbaugh). The University of Arizona has carbon dated it to have been made between 1404 and 1438, though otherwise everything about the book, it’s author, illustrator, the text, and its language, are a complete mystery.
The writing in it matches no known language and has been of interest to linguists and code breakers since the early 1920’s. Theories of the language’s origin run the gambit from being an unknown relative of East-Asian writing to the ramblings of a mad man to William Newbold’s disproved “micrographic hypothesis” where the manuscript would have been written with a powerful series of magnifying lenses using microscopic letters which read normally when enlarged but appear as unrecognizable/nonsensical letters to the naked eye.
The Voynich Manuscript also contains botanical illustrations of plants that don’t exist, star charts that make no sense, and scenes of naked women bathing in green liquid that has traveled through a series of maze like pipes.
Once thought to be an alchemic manual for an elixir of life (likely due to its earliest known ownership by Georg Baresch, a self proclaimed alchemist in Prague during the early 17th century) the Manuscript could also possibly be an attempt to smuggle coded military and engineering secrets into the Ottoman Empire.
Frankly, given how much the book has traveled in its last six hundred years I’m amazed it hasn’t been destroyed.
Please also look at the book A Humument by Tom Phillips, which defies classification (though technically it’s a “treated Victorian novel” I don’t think that’s a category on Amazon.com. The best classification I’ve heard was when Tom Nozkowski referred to it as a “Non-Art Book”) and Luigi Serafini’s (possibly Voynich inspired) Codex Seraphinianus.