After The Golden Triangle, the next Arsene Lupin novel published by Maurice Leblanc was L'Île aux trente cercueils, The Isle of Thirty Coffins, which seems often to be published in English under the title, The Secret of Sarek (Sarek being the name of the fictional island) -- perhaps publishers feared the original title gave away too much. In any case, it was published in 1919, the year after The Golden Triangle, first in serialized form in Le Journal and then later in the year as a book. It occurs in 1917, primarily focusing on the heroine of the story, Véronique d'Hergemont, and a prophecy that comes true. And that is almost all that I have been able to find about it, so we shall see what it is like.
