For example, The Voynich Manuscript:
The Voynich manuscript might be the most unreadable book in the world. The five hundred year-old relic discovered in 1912 at a library in Rome, consists of 240 pages of illustrations and writing in a language not known to anyone. Deciphering the text has eluded even the best cryptographers and some have even dismissed the book an entertaining but lengthy hoax. But a statistical analysis of the writing shows that the manuscript does seem to follow the basic structure and laws of a working language.
Link. Voynich at Wikipedia.