Zara S. Steiner, who wrote deeply researched books on the origins of World War I and the period between the world wars, adding new layers to the standard discussions of diplomacy and its shortcomings, died on Feb. 13 at her home in Cambridge, England. She was 91.

Her son, David, said the cause was pneumonia.

Dr. Steiner was unusual not only because she was a woman in a male-dominated field but also because she was an American whose writing largely centered on Britain. She was married to the essayist and scholar George Steiner, who spent much of his career at Cambridge University. He died 10 days before she did.

Dr. Steiner dug into the letters and other papers of civil servants and interviewed prominent and not-so-prominent historical players to flesh out the picture of how momentous events were shaped. Her books looked not just at the standard official documents, but at how unheralded staff members, public opinion and more propelled history.