They have been spotted prowling the damp streets of Dublin, poring over dog-eared editions of James Joyce's "Ulysses," and crowding outside the gate of Peter Mayles's home in France, brandishing copies of "A Year in Provence."
Nowadays, new incarnations of literary tourists are descending in droves on Paris, intent on cracking "The Da Vinci Code" by following in the footsteps of the heroes and villains in Dan Brown's bestselling historical murder mystery.
From the shadows of Saint Sulpice Church, parish priest Paul Roumanet looks on indulgently as a knot of sightseers examine a rod of brass inlaid into the flagstones of the ancient floor that plays a key part in the plot. "For many of them it's a nonreligious pilgrimage," he says. "And lots find it hard to understand that the book is fiction."
Few who take the various "Da Vinci Code" tours of Paris these days can fail to spot that the book blends fact with fiction. But as they explore the line that separates the two, they are feeding a boom in the literary corner of the travel market that seems to soothe a postmodern malaise.
"It is the existential doubt that people have about modern experience" that drives them beyond the covers of a book to the places that inspired them, says Phil Cousineau, a writer and filmmaker who himself leads literary tours to Europe. "You can read a book or see a movie, but you're not quite sure it's real until you've been there."
Ellen McBreen, a Paris-based art historian and tour guide agrees. Following a book, she says, "gives travel more depth, and everybody wants something that feels more real and more authentic, even if, ironically, it is based on fiction."
Ms. McBreen runs lively but academically rigorous tours of the Louvre, where Mr. Brown's book opens, examining the evidence for the book's thesis that the Holy Grail was not a cup, but actually Mary Magdalene, the bride of Jesus, who bore his children and carried his bloodline, which ran to the early kings of France.
McBreen starts her tour outside the museum by a Napoleonic triumphal arch, so as to recall Napoleon's quip - quoted in the book - "What is history, but a fable agreed upon?"
"The idea that there is no absolute historical truth, that it depends who is writing the history, is a bit unsettling but exciting to a lot of my clients," she says.
In ages before such relativism became accepted, pilgrims followed only in the footsteps of saints. Today, suggests Mr. Cousineau, travelers also seek out the haunts of artists and writers in order to satisfy "the urge to take things to a deeper level, because so much of life is superficial and vicarious."
There is no more luxurious way to escape the superficial and vicarious while seeking out the secrets of "The Da Vinci Code" than to stay at Olivia Hsu Decker's 17th-century chateau outside Paris.
A week there, including breakfast and tours of all the spots mentioned in the book (including the Ritz Hotel, of course) will set a couple back $6,900. But they will have the satisfaction of being part of the plot: the Chateau de Villette appears in the book as the home of an eccentric English expert on the Holy Grail.
Ms. Decker, a San Francisco realtor convinced by the book's premise, has already run a couple of tours of her chateau for people she says who "come here on their own search for the truth" about Mary Magdalene.
Some of them, she adds, seem a little confused about the line between fiction and reality. "They wanted my autograph," she recalls. "I told them I didn't write the book, I just own the chateau, but they insisted."
That happens with other literary Meccas, too. The baseball diamond carved out of a cornfield in Dyersville, Iowa, where the film "Field of Dreams" was shot, has nothing to do with "Shoeless Joe" Jackson, the ballplayer whose story was told in a book of the same name by W. P. Kinsella, which was turned into the film. But the movie site still attracts thousands of fans a year, especially from Japan.
Prince Edward Island in Canada has been transformed by "Anne of Green Gables," whose author, Lucy Maud Montgomery, lived there. Summers in the small town of Cavendish - which boasts a house said by the tourist authorities to have inspired "Green Gables" - are now a far cry from bucolic calm depicted in the book as hundreds of thousands of sightseers visit.
Getting too real carries its own dangers, though. Peter Mayle was hounded from his home in Provence when he made the mistake of describing its location too precisely in his first book, "A Year in Provence." Fed up with coming in from his garden to find perfect strangers sitting on his living room sofa demanding an autograph, he fled with his wife to America for several years.
If the busloads of tourists in Provence were there simply because they were curious to see a place that had captivated them in print, others are drawn elsewhere by "fascination with the spark of creativity," suggests Cousineau.
That would explain the groups of tourists standing around outside the nondescript patent office in Bern, Switzerland: Albert Einstein is said to have come up with the Special Theory of Relativity while he was working there in 1905.
Others find magic in the Long Island, N.Y., birthplace of "Leaves of Grass" author Walt Whitman. Still others pay to take a tour of Silicon Valley that includes such sacred sites as the garage in Palo Alto, Calif., where William Hewlett and David Packard began working together in 1939.
"People like to go to where it all began," says Cousineau. "Perhaps it's because we feel we might share in the genius, that a little bit might rub off on me."
Meanwhile back in Saint Sulpice, Father Roumanet is beginning to grumble about the 15,000 extra visitors who have passed through his church in recent months because of "The Da Vinci Code."
In June he posted a tartly worded notice near the brass line, pointing out that "contrary to fanciful allegations in a recent best-seller, this is not a vestige of a pagan temple" as the book claims, but part of a 17th-century scientific instrument to measure the earth's orbit.
"The misfortune of our contemporaries," the priest sighs, paraphrasing the English Catholic poet G.K. Chesterton, "is perhaps not that they have ceased to believe, but that they are ready to believe anything."