PARIS — “The 120 Days of Sodom,” by the Marquis de Sade, is one of the most perverse works of 18th-century literature.

It tells the story of four rich “libertines” who lock themselves in a remote medieval castle with 46 victims (including eight boys and eight girls, ages 12 to 15). The men are assisted by four female brothel keepers who arouse their hosts by recounting their outlandish (and embellished) experiences....

Even Bruno Racine, director of the Bibliothèque Nationale de France, the National Library, calls it “depraved.”

But that hasn’t stopped him from negotiating long and hard to buy Sade’s manuscript. He has convinced the Foreign and Culture Ministries of its importance. He has argued in front of the Commission of National Treasures to declare it provisionally a “national treasure” that needs to be preserved in the library. And he is ready to pay more than $5 million to get it....