Maurice Leblanc 
The Golden Triangle [EPUB ebook] 

The Golden Triangle (also known as The Return of Arsène Lupin) was published in 1917 in both the original French and this English translation. It is representative of its time with themes of convalescent soldiers and continent-wide plots.
Captain Belval, learning of a threat to his beloved nurse Little Mother Coralie, rescues her from her would-be assailants and is promptly dragged into a plot involving her husband and millions of francs worth of gold. As layer upon layer of conspiracy emerges with no obvious thread to follow, there’s only one man who can be counted on to uncover the truth.