Surely this is finished, if someone gets the missing title the winner will be the one who has already given two titles.
I concur; just -- have been out, between shortly after my 0706 post; and a little while ago -- "for completeness", saw giving it a little while, in case someone might roll up with the final title (as with @MotCO, immediately preceding post, "might have, but didn't").
Missing one: The Three Hostages, published 1924, action taking place approximately then.
@Gloster: your turn.