Birkin, who died in 2023, had “it”: an undefinable, unmistakable glamour that shifts our collective sense of what’s cool, or at least of what’s worth paying attention to. Easily mingling English reserve and European sensuality, she had a sweetness that set her apart from contemporaries such as the bombshell Brigitte Bardot or the edgier Anna Karina. “She wasn’t a hippie,” the journalist Marisa Meltzer writes in her new biography, “It Girl: The Life and Legacy of Jane Birkin” (Atria), “but rather a rising star from the upper class,” someone who radiated privilege even when she dressed down. One of the first celebrities to be regularly photographed in her everyday clothes, Birkin was an early icon of street style, traipsing around Paris in sneakers and rumpled sweaters, wicker basket in hand. The outfits could be easily mimicked and therefore easily marketed. Today, when social-media influencers praise “the French-girl look”—wispy bangs, minimal makeup, bluejeans, marinière tops—the look they have in mind is hers.