Codex Seraphinianus

This book actually exists in our world (believe it or not) but is so weird that it would probably be more believable in roleplay world where it had popped through a magic gate to another world. Who knows, maybe in a role play world, the bizarre things it depicts can actually be reached. (Incidentally, I haven't read it, though I would like to at some point) - it's quite hard to get hold of.

Codex Seraphinianus

Essentially a fictional encyclopedia, depicting the bizarre fauna, flora, geography, culture and history of an imaginary world. Covered with detailed illustrations and scientific drawing and annotated in a bizarre fictitious language and a bizarre numbering system (mainly base 21 with some added weirdness; also some base 27). To make things even more bizarre, at one point there is a 'Rosetta stone' depicted in the book - however, this just translates the fictitious script in which the book is written in to another, even weirder fictitious language of hieroglyphics.

The book is divided in to sections (e.g. fauna, history, language). To quote someone who's read it:
'After sections on flora and fauna, there is one on a genus consisting in of pairs of legs, which support not bodies but various surprising terminations such as umbrellas, balls of yarn, striped pods out of which tigers burst when ripe, numerous free-form shapes. There is a relatively abstract section on natural (or from our point of view unnatural) elements, along with scientific machines for analyzing them. There is a section depicting variations on parts of the human body as these are exhibited by sundry species.

There is a section on races, civilizations and customs of the countries shown on a map of the world, a world whose continents and islands seem to have the ability to rearrange themselves at will; there are sections on food, clothing, games, and urban architecture. Perhaps the strangest section is the one on language, where the materiality of the letter is taken to the extreme. Here letters and words proliferate into different species beyond our own world's superficial differences of language.'

http://www.io.com/~iareth/codindx.html
http://faculty.msvu.ca/pschwenger/codex.htm
http://www.math.bas.bg/~iad/serafin.html