This is a rather bogus word which, perhaps because of its rarity, has managed to survive. It originated as a description, in Latin, for books originating from the new-born art of printing, incunabula (neuter plural the Oxford English Dictionary points out — the putative singular form “incunabulum” is not found in Latin) meaning swaddling clothes; cunae apparently being cradle. Converted into the back-formed noun “incunabulum”, plural “incunabula”, its definition has narrowed down to indicate only books printed before 1501. The OED‘s earliest quote from J. M. Neale in 1861 attributes the usage to the Germans. English-ing it to incunable [in-queue-nable] is pretty well established now, though the OED credits that word to the French. The vagueness surrounding the word may encourage many to resort to “incunabula” as a singular, which at least protects you against the aural ugliness of incunable. In a world of total confusion, nobody can tell you you’re wrong whichever form you chose to use when talking about books printed before 1501. Maybe just saying “books printed before 1501” would be a safe and euphonious choice.

Teasingly the OED tells us incunabula also means “the breeding-places of a species of bird”. They support this with no quotes, and leave unclear whether they mean any species or one special species of bird.