In 1975, when Nelson DeMille was in his early 30s, he published his first full-length novel, The Quest, with Manor Books. The book hinges on one of history’s most enduring secrets: the location of Christ’s cup from the Last Supper, the Holy Grail.

The search for the Holy Grail “has universal appeal in Western culture,” said DeMille. The continuing fascination with the Holy Grail was proven once again when DeMille’s revamping of the novel, released on September 17, debuted at #5 on PW’s adult hardcover fiction list and last week was in the seventh spot, with total sales of just under 42,000 copies at outlets that report to Nielsen BookScan. Yet, DeMille was motivated to dust off a book he wrote nearly 40 years ago by more than the evergreen allure of the Holy Grail.

“I always wanted to [rewrite this book]. It was my best of the paperback originals I was writing in the ’70s,” said DeMille, characterizing the story as Indiana Jones meets The Da Vinci Code. DeMille first tossed around the idea of revisiting the novel about 10 years ago, but it wasn’t until fall of last year that he signed a contract for a “modest advance.” Because the rewrite was different from his usual projects, rather than work with his longtime publisher, Grand Central, DeMille signed with another Hachette imprint, Center Street. He didn’t specify the size of the deal, but admitted that it was more than the $1,200 he received from Manor Books in 1975. DeMille began the rewrite last February, contending with a manuscript shaped by his younger self.

“I was a 30-year-old writer and I really didn’t know that much about writing novels,” said DeMille. He found that the structure still held up, but paid special attention to key scenes that “needed to be milked a little more.” The novel doubled in size, stretching from 75,000 to 140,000 words. DeMille described the original edition as essentially a “long outline,” and noticed one advantage to working with an existing book right off the bat. “I didn’t have to spend all that time with the research and plotting structure,” he said. “I was really able to concentrate on the characters and the writing.”

DeMille set out to bring the novel up to date by making other changes that reflected the sentiment of 2013, rather than 1975. “The biggest difference is the ending in the [1975] edition,” he said. “It was a real downer, and the ending in the 2013 book is much more upbeat. I ascribe it to the down feeling of the ’70s. That was not an unusual ending then—it was just that kind of time. It was a grim story before I lightened it up a bit.”

He also accommodated the evolution of his readership in the new version of the novel. “When I was writing these books back in the ’70s I did muscular N.Y.P.D. stuff,” said DeMille, adding that since his audience is now a little over half female, he turned up the volume on the love story. “I added more sex to the book, which may or may not have helped the situation,” he noted.

For DeMille, the project carries personal significance in terms of his journey as a writer. The main character, Frank, is 30 years old—DeMille’s age when he first published The Quest. Another character in the book, Henry, is in his late 60s. “I’m more Henry’s age than Frank’s age,” said DeMille, who recently turned 70. “I can now see a perspective from both sides. I got better into Henry’s head, and I can look back at Frank. It’s kind of a flashback in time to where I was, to who I was.”