The Club Dumas by Arturo Perez-Reverte

A mystery for bookies only. Corso is hired by avid book collectors in Europe to verify the authenticity of rare and ancient books. In this case, a chapter of The Three Musketers in Dumas handwriting and The Nine Doors (theoretically destroyed during the Inquisition because of its connection to the Devil), are the objects of his search. The story gets darker and darker and the ending is a complete surprise. A bit pretentious in it's literary references - in the style of Umberto Eco.