Found as serial number 21232-7 in a 1969 panel advertisement by the publisher --NY Times 1969-04-13 pBR31, "Off-trail paperbound books from Dover, at lowest prices" with order form-- wherein the listing for this book suggests a long title with "Three Gothic Novels" as subtitle.
Emphasis original, there with component titles all caps:
The Castle of Otranto, Horace Walpole; Vathek, Wm. Beckford; The Vampyre, John Polidori: Three Gothic Novels (with a Fragment by Lord Byron), E. F. Bleiler, ed.
The headings for other listings are such as (serial number 20487-1):
The Devil's Dictionary, Ambrose Bierce.
WorldCat library records display various heading/titles including Three Gothic Novels and [list of 3] and Three Gothic Novels: [list of 3] and [list of 3]: Three Gothic Novels and longer --where "[list of 3]" is fashioned variously.