(With thanks to Philippe Aurousseau, & Courtesy of Oncle-Archibald.blogspot.fr)
Aptly and obviously for a genre identified with the resolution of an enigma, Detective novels have often been marketed with big interrogation marks on their covers. One of the earliest Crime Fiction Series, Ferenczi’s booklets “Le petit roman policier” was recognizable for the question mark adorning its covers designed by Gil Baer.
Later, two well-known French Crime Fiction series at least were named “The Question Mark”.
1/ The Editions Pierre Laffitte’s series “Le Point d’interrogation” was published in Paris from 1932 and until 1937. This series was devoted almost entirely to Gaston Leroux and Maurice Leblanc.
2/ The Hachette Series “Le Point d’Interrogation” was published between 1951 and 1965.
Countless more series and stand alone crime fiction books used visual references to question marks. If you know about such series or book covers from your own countries and in any languages, please let us know about them !