Codex Seraphinianus

From Wikipedia:

The Codex Seraphinianus is a book written and illustrated by the Italian architect and industrial designer Luigi Serafini during thirty months, from 1976 to 1978.[1] The book is approximately 360 pages long (depending on edition), and appears to be a visual encyclopedia of an unknown world, written in one of its languages, an incomprehensible (at least for us) alphabetic writing.

A more in depth look at the codex with more illustrations can be found here:

One day Dr. Harpold came to class visibly excited. He said he had found a very rare, delicate, and expensive book just sitting on the shelf at the university library. It was typical, he said, because the few libraries that owned a copy of this rare book didn’t know how valuable it had become, so it got shelved with the general collection and subsequently stolen by the first savvy person who came upon it. Harpold had caught our university library’s grievous error and had had it corrected, but not before pulling faculty privilege and checking it out himself. After admonishing us to make sure our hands were clean, he passed it around. “It” was a book called Codex Seraphinianus, by one Luigi Serafini, published in an extremely limited edition in Italy in 1981. The book was an oversize black hardback. The cover art was a vaguely encyclopedic depiction of a man and a woman engaged in successive stages of copulation, then melding together, and finally becoming a single alligator.