I nominate two books:
-Gulliver's Travels by Jonathan Swift
From Goodreads:
Shipwrecked castaway Lemuel Gulliver’s encounters with the petty, diminutive Lilliputians, the crude giants of Brobdingnag, the abstracted scientists of Laputa, the philosophical Houyhnhnms, and the brutish Yahoos give him new, bitter insights into human behavior. Swift’s fantastic and subversive book remains supremely relevant in our own age of distortion, hypocrisy, and irony.
There are many versions available in the PCML (I counted 15 if you search "Swift Gulliver" in the library section), including many formats and even versions in German and Romanian, so for brevity I'll only link to a few:
Illustrated epub uploaded by our own Hamlet53 (this version also available in imp)
Kindle version uploaded by Madame Broshkina (this version also available in imp and bbeb/lrf)
PDF uploaded by Jellby (this version also available in epub)
German epub uploaded by netseeker (this version also available in kindle and bbeb/lrf)
-Le Morte Darthur by Thomas Malory
From Goodreads:
An immortal story of love, adventure, chivalry, treachery and death. Edited and first published by William Caxton in 1485, Le Morte D'Arthur is Sir Thomas Malory's unique and splendid version of the Arthurian legend. Mordred's treason, the knightly exploits of Tristan, Lancelot's fatally divided loyalties and his love for Guenever, the quest for the Holy Grail; all the elements are there woven into a wonderful completeness by the magic of his prose style.
The result is not only one of the most readable accounts of the knights of the Round Table but also one of the most moving. As the story advances towards the inevitable tragedy of Arthur's death the effect is cumulative, rising with an impending sense of doom and tragedy towards its shattering finale.
Some links to PCML versions:
epub uploaded by weatherwax
Kindle version uploaded by our own BenG