Earlier this week, before the attack in Paris, President François Hollande said he would read it. Marine Le Pen, the leader of the right-wing National Front, whom the fictional Muslim leader defeats in the novel, cashed in, calling it “a fiction that could one day become a reality.” Late Thursday, Agence France-Presse reported that Mr. Houellebecq (pronounced WELL-beck) had stopped promoting the book.

Image The cover of the current issue of the French satirical newspaper Charlie Hebdo.

With an ambitious initial print run of 150,000 copies, “Submission” is already the No. 1 seller on Amazon in France. It is likely to join another book with a similar theme on best-seller lists: “The French Suicide,” a 500-page essay in which the journalist Éric Zemmour, 56, argues that immigration, feminism and the 1968 student uprisings set France on a path to ruin. The top seller in France, the book has sold 400,000 copies since its release in October, according to its publisher, Albin Michel.

Though Mr. Zemmour’s is a work of reactionary nostalgia and Mr. Houellebecq’s a futuristic fantasy, both books have hit the dominant note in the national mood today: “inquiétude,” or profound anxiety about the future.

Fueling this anxiety for many French are the fears of non-Muslims about Muslims, the threat posed by groups like the Islamic State and their recruiting in Europe, and rising anti-Semitism. More broadly, concern has grown that the political center is eroding and that extremes are rising in a way reminiscent of the 1930s, along with a sense that France, which prides itself on its republican tradition and strong, centralized state, has ceded too much power to the European Union.

“I think this anxiety is the idea of seeing France give up on itself, of changing to the point of no longer being recognizable,” said the philosopher Alain Finkielkraut, whose much-debated 2013 book, “L’identité Malheureuse,” or “The Unhappy Identity,” discussed the problems immigration poses for French identity and cultural integration. “People are homesick at home,” he added, speaking two days before the attacks.