Photoplay editions were usually hardcover reprints of novels that had been made into movies, illustrated with photographs from the film productions. Sometimes, instead, they were "fictionized" versions of film scripts, rewritten in narrative form. Here is an annotated checklist of more than 500 horror and mystery photoplay novels and magazine fictionizations, collected over a period of four decades. Photo-illustrated stories that are not strictly in the horror or mystery genres are included if they are linked to films with such stars as Lon Chaney, Boris Karloff, Bela Lugosi, or other genre figures. Mysteries are generally defined as novels or stories featuring a detective as the central character, although in some cases melodramas, thrillers, and film noir books having crime as a plot element are included. Science fiction and fantasy works, and others having outr aspects, are also within scope. With a few exceptions, the cut-off date for inclusion in the catalog is the year 1970. In an entertaining introductory essay the author reflects on the attractions of assembling such a collection, analyzes aspects of the social significance and aesthetic content of its books, and draws many surprising inferences from their advertisements, illustrations, and marks of previous ownership. The subsequent catalog is the first survey in the field to extend bibliographical coverage beyond books to movie tie-in magazine stories. Included in an appendix is the complete text of "The Gorilla, " a short story version of a lost First National Film, reprinted from a rare issue of Moving Picture Stories from 1927.