Book Three of the Hallowe'en Nights trilogy was published in 1938 in the United States as Behind That Mask. The first two books were published as one book, Finger! Finger! All three had been published in 1938 in Britain as one huge mega-novel, entitled Behind That Mask. Keeler did his damnedest to make this book into a mystery novel that would stand alone, but you really should read Finger! Finger! first.