16. Codex Seraphanius
An Italian architect and designer drafted and wrote the Codex over around 30 months in the late 70s, creating an encyclopedia for another world of alien biology and strange text which has never been deciphered. Borges would be proud. At 360 pages and 11 sections long, it's logically laid out and batshit bonkers, and every attempt to figure out what the writings say have hit nothing but walls. I would put this further down the list, but author Luigi Serafini recently admitted it was untranslatable. While it has a real number system (in base 21), the words are meant to have an obvious form and function, but not be understandable, and he wanted " the alphabet to convey to the 'reader' is the sensation that children feel in front of books they cannot yet understand, although they see that their writing does make sense for grown-ups."