This word — meaning a book published before the beginning of the sixteenth century — is better known in its plural form incunabula. Though rare, it survives because there is so much interest in early printed works. It conjures up an image of shelves of dusty and ancient books, no doubt overseen by dusty and ancient collectors.
The odd thing about this word is that originally it had nothing whatever to do with books. It derives from a Latin word incunabula, plural like its English descendant, meaning the swaddling bands that held an infant in the cradle (if you trace it back further still, you arrive at the Latin root cunae, cradle). Even in Latin, the word had a figurative sense of infancy in general, very much as we use cradle in phrases like “The cradle of the Industrial Revolution”.
Incunabulum is a fairly recent import into English, appearing only at the beginning of the nineteenth century. At this stage it could refer to the early stages in the development of anything. Thomas De Quincey — the author of Confessions of an Opium Eater — is its first recorded user. But the first person to apply it to early printed books, those created in the infancy of the art, was John Mason Neale, in his splendidly entitled Notes Ecclesiological and Picturesque on Dalmatia, Croatia, Istria, Styria, with a Visit to Montenegro of 1861, though he makes it clear he was borrowing it from German.
A variant form of the singular is incunable and someone who collects such works is an incunabulist.