Early Experimental Literature

Two examples of Post-Modernism from very Pre-Modern times:



The Voynich Manuscript
Nobody has ever decyphered the glyphs that fill the pages of the undateable Voynich Manuscript, leading some scholars to conclude its hundreds of pages are just dense gibberish. It could be an encyclopedia from an alternate Earth, with fantastic botanical and astronomical studies and revelations.  The colorful illustrations only make the enigma more complex, with impossible to pin-down imagery.
Look it up in the wikipedia.
See its pages at the Beinecke Library.
There are several published studies of the Voynich Manuscript in book form, also: the Kennedy-Churchill book being the most contemporary, and seemingly least biased towards particular theories.

Hypnerotomachia Poliphili
And, for another book far outside its--or any other-- time, the Hypnerotomachia Poliphili, or "Strife of Love in a Dream,"
please refer, again, to your wikipedia.
The pages can be viewed at MIT Press.
This one-of-a-kind proto-novel was written in an embellished, eccentric Latin, framed in Italian-style syntax, told from multiple, embedded frames of reference.  The story follows a hero on his dream-walk tour of fantastic--sometimes erotic--architecture, cataloging ambitious remixes of Classic details, and gardens that describe vast poetic metaphors.  It was published in 1499, only five decades after of the European invention of the printing press.
Translations of the Hypnerotomachia Poliphili (I learned to say it--so can you!) are published from time-to-time, and there are numerous studies available in book form.