I don't think I've seen an earlier book with an illustrated cover. I'd love to hear of any examples.
Horace Walpole, ll Castello di Ortranto. Storia gotica. Trans. Jean Sivrac (In Londra : presso Molini, Polidori, Molini e Co. Hay-Market; Ed I. Edwards, Pall-Mall, 1795). Kenyon Spec. Coll.
4 months ago