Wasn't majorly aware of the Voynich one and the amount of time it has remained uncracked for, until I just read an article about someone who thinks he has cracked the code to the book, which has baffled cryptographers and codebreakers for nearly 600 years!
I've 'read' the Codex Seraphinianus (I can upload a .pdf of this if anyone is interested), and it's ingenious and beautiful to look at. It actually sort of makes sense, in a weird way. Not necessarily in words and a 'story' as such, but the flow of it, and what it is referring to throughout the pages.
Love stuff like this, the Seraphinianus is basically an imaginary language telling of about an imaginary world, and it's incredible to see... totally bonkers of course!