The Man in the Iron Mask by Alexandre Dumas.
 
The strange rumour of a high-born, closely guarded political prisoner whose face was concealed by an iron mask ran through Europe during the reign of King Louis XIV of France.
 
Dumas, with masterly skill, weaves a brilliant story of intrigue, love and mystery and focuses it upon the identity of this unhappy captive.
Here is a plot unfolded so subtly and swiftly and with such drama and realism that the reader is kept on the tiptoe of excitement and suspense.