And unlike France’s National Library, whose sexually explicit material is contained in one archive, only a part of the Public Library’s erotica was designated triple star. The rest is dispersed in other collections in the building, including in the Berg Collection of English and American Literature (rare books and manuscripts) and the Spencer Collection (artists’ books and illuminated manuscripts).

A guided visit to the library revealed some of the richness of its erotic (or pornographic, depending on who was doing the classification) material. The works are hidden treasures, many of them awaiting discovery. Not even the curators and librarians know everything that is there.

“There were many materials in the library’s special collections that I had never seen before,” Mr. Baumann said. “The range and depth of our collections never ceases to astonish me.”

The main building of the Public Library had such an impact on the neighborhood that there was once a massage parlor a block away on West 43rd Street named the Library. A 1976 flier in the *** collection advertised its $10, tip-included service, with “7 Beautiful Librarians to Service You.” The flier shows a longhaired “librarian” dressed in a necklace and high heels. A large bunch of feathers covers her private parts.

As part of the library’s mandate to collect life as it was lived, small teams of librarians were dispatched in the 1970s to Times Square pornography shops to scoop up representative samples of the latest erotica. Among the paperback titles in the collection: “Animal Urge,” “The 48-Hour Orgy,” “Beach Stud” and “All Day Sucker.”