The Club Dumas by Arturo Perez-Reverte

(From Goodreadseads)

Lucas Corso is a book detective, a middle-aged mercenary hired to hunt down rare editions for wealthy and unscrupulous clients. When a well-known bibliophile is found dead, leaving behind part of the original manuscript of Alexandre Dumas's The Three Musketeers, Corso is brought in to authenticate the fragment. He is soon drawn into a swirling plot involving devil worship, occult practices, and swashbuckling derring-do among a cast of characters bearing a suspicious resemblance to those of Dumas's masterpiece. Aided by a mysterious beauty named for a Conan Doyle heroine, Corso travels from Madrid to Toledo to Paris on the killer's trail in this twisty intellectual romp through the book world.