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.

The Encyclopedia of the Brethren of Purity

The Encyclopedia of the Brethren of Purity, written by a mysterious band of philosophers in 10th Century Basra in what is now Iraq. It is generally considered the original encyclopedia.

"...to shun no science, scorn any book, or to cling fanatically to no single creed. For [their] own creed encompasses all the others and comprehends all the sciences generally. This creed is the consideration of all existing things, both sensible and intelligible, from beginning to end, whether hidden or overt, manifest or obscure . . . in so far as they all derive from a single principle, a single cause, a single world, and a single Soul."
- from the Ikhwan al-Safa, or Encyclopedia of the Brethren of Purity; Rasa'il IV

The Brethen divided the sciences into three categories, outlined here, with help from Professor De Callata├┐ at the Institure of Ismaili Studies in London.

Propaedeutic Science: the sciences of education, which serve the quest of subsistence and for the goodness of the living in this world, including: writing and reading, language and grammar, calculation and operations, poetry and prose, auguries and auspices and the like, magic, talismans, alchemy, tricks and the like, professions, crafts, sale and purchase, trades, cultivation, breeding, biographies and histories.

Religious and Conventional Science: the sciences of the healing of souls and for the quest of the hereafter, including: science of revelation, science of interpretation, narratives, reports, jurisprudence, norms and laws, recollection, exhortations, asceticism and mysticism, interpretation of dreams.

Philosophical Sciences:
Mathematics: arithmetic, geometry, astronomy, music
Logic: poetics, rhetorics, topics, analytics, sophistics
Natural Sciences: science of corporal principles, science of the heaven and the world, science of coming-to-be and passing-away, science of atmospheric events, science of minerals, science of plants, science of animals
Metaphysics: knowledge of the Creator, science of spiritual beings, science of psychic beings, science of politics (with 5 subdivisions: prophetic, royal, public, domestic, private), science of the Return