They touch on subjects ranging from the novel’s often obscure literary references and suggested further reading on “the use of impatiens blossoms and garlic juice to dye women’s fingernails” to obscure Ming-era slang whose meaning, Mr. Roy notes with pride, had long eluded even native Chinese-speaking scholars.

“It’s not just a translation, it’s also a reference book,” said Yihong Zhang, a visiting scholar at the University of Pittsburgh who is translating some of Mr. Roy’s notes into Chinese as part of his doctoral dissertation at Beijing Foreign Studies University. “It opens a window onto Chinese literature and culture.”

And then there is the sex, which has fed fascination with the book, even though few people could actually read it. In Mao’s China, access to the unexpurgated edition was restricted to government high officials (who were urged to study its depiction of imperial corruption) and select academics. Today, complete versions remain hard to find in China, though it is easily downloadable on Chinese Internet sites.

The level of raunch remains startling even to some Western literary scholars — particularly the infamous Chapter 27, in which the merchant, named Ximen Qing, puts his most depraved concubine to particularly prolonged and imaginative use.

“When I taught it, my students were flabbergasted, even though they knew about the novel’s reputation,” said Patricia Sieber, a professor of Chinese literature at Ohio State University. “S-and-M, the use of unusual objects as sex toys, excessive use of aphrodisiacs, sex under all kinds of nefarious circumstances — you name it, it’s all there.”

The novel’s sex has also inspired some modern reconsiderations. Amy Tan’s new novel, “The Valley of Amazement,” features a scene in which an aging courtesan in early-20th-century Shanghai is asked to re-enact a particularly degrading sex scene from this classic.

“I can’t say any of the characters are likable,” Ms. Tan said of the older novel. “But it’s a literary masterpiece.”