The Codex Seraphinianus was published in 1981 by Luigi Serafini and shows art from an imaginary world created in the 1970s. The art is surreal, often a parody of a real life creature and sometimes slightly disturbing. Along with illustrations there’s a mysterious writing system which has become a challenge and a matter of debate among linguists.
Original editions are now rare and expensive but a 2006 edition was released in Italy and is much cheaper.
It’s a book that will appeal to some GMs in their search for inspiration and oddness for their games. For example; what if there are no mermaids in your fantasy world – but rumours of them persist? What would a good fantasy RPG explanation for that be?
Hit up Amazon for the book.