PARIS — Georgette Elgey, a French journalist, editor and historian best known for her six-volume history of France in the years after World War II, a project that took her nearly a half-century to complete, died on Oct. 8 in Paris. She was 90.

In a statement confirming her death, President Emmanuel Macron of France called Ms. Elgey “one of the greatest experts of the Fourth Republic.”

Ms. Elgey had started her career as a journalist, but she found more fulfillment in researching and writing about France’s Fourth Republic, a complex era that lasted from 1946 to 1958. She embarked on the project in the early 1960s and labored on it into the 21st century, delving deep into archives and drawing from oral testimonies. The first volume appeared in 1965, the final one in 2012.

Ms. Elgey’s “Histoire de la IVe République ” explored a period marked as much by promise as by uncertainty. France experienced strong economic growth in the immediate postwar years under the Marshall Plan, the American effort to put a ravaged Europe back on its feet, and rebuilt its relations with Germany, a reconciliation that led to the formation of the European Union.