Le Morte Darthur

First published in 1485 by William Caxton. A different version of the text was discovered in 1934 (Winchester Manuscript); it was published in 1990 as part of Malory's The Works by Oxford University Press.
There are only 14 known copies of the first 4 editions, i.e. before the 1634 edition. (Pollard & Redgrave, Short Title Catalogue of Books Printed in England, Scotland & Ireland)