In his mid-20s when we meet him in 1939, Camus was a hugely ambitious, if yet to be published, writer living in his native Algeria. He was working on a novel he would abandon titled “A Happy Death”; a play about the emperor Caligula; and a philosophical essay, “The Myth of Sisyphus.” He would soon add the germinal ideas of “The Stranger” to that mix.

Image Alice Kaplan Credit... Catherine Hélie/Gallimard

Camus, though, as is well known, was a man involved in the world, not a writer locked in his room, and his story is deeply entwined with the complex political climate in French-ruled Algeria during the time that France was occupied by the Nazis. Ms. Kaplan, a professor of French at Yale, is the acclaimed author of several previous books, including the memoir “French Lessons” and “Dreaming in French: The Paris Years of Jacqueline Bouvier Kennedy, Susan Sontag, and Angela Davis.” To this new project, she brings equally honed skills as a historian, literary critic and biographer.

We learn extensively about Camus’s relationships with his family and with fellow writers; about his activities as an anti-fascist; and about what led to everything in “The Stranger,” from the rhythm of its sentences to the conception of the unforgettable scene in which a sun-dazed Meursault murders an Arab on the beach.

Indicative of Ms. Kaplan’s approach is the chapter on Camus’s time as a court reporter for the anticolonial newspaper Alger-Républicain in the late 1930s. The experience gave him a front-row seat in “a theater for the tensions and dramas of a society structured on inequality.” It also provided him specific material for “The Stranger,” like the scene in which a judge waves a crucifix at Meursault and insists that he believe in Christian forgiveness.

“The Stranger” was the first work of fiction to fully convey the icy alienation of existentialism. (Camus didn’t like the E word, but the shoe fits, and snugly.) This slim, spartan novel’s antecedents were garishly voice-driven by comparison. The unnamed narrator of Dostoevsky’s “Notes From Underground” has an often spirited, wisecracking relationship with the reader. (“In my soul I have never been a coward, though I constantly turned coward in reality, but — don’t laugh too quickly, there’s an explanation for that; rest assured, I have an explanation for everything.”) The protagonist of Sartre’s “Nausea,” published in 1938, just four years before “The Stranger,” is more akin to Dostoevsky’s garrulous, opinionated guide than to Camus’s detached antihero. (“To think that there are idiots who get consolation from the fine arts.”)