Sixty years ago, at the height of his powers, Frank left New York in a secondhand Ford and began the epic yearlong road trip that would become ‘‘The Americans,’’ a photographic survey of the inner life of the country that Peter Schjeldahl, art critic at The New Yorker, considers ‘‘one of the basic American masterpieces of any medium.’’ Frank hoped to express the emotional rhythms of the United States, to portray underlying realities and misgivings — how it felt to be wealthy, to be poor, to be in love, to be alone, to be young or old, to be black or white, to live along a country road or to walk a crowded sidewalk, to be overworked or sleeping in parks, to be a swaggering Southern couple or to be young and gay in New York, to be politicking or at prayer.

The book begins with a white woman at her window hidden behind a flag. That announcement — here are the American unseen — the Harvard photography historian Robin Kelsey likens to the splash of snare drum at the beginning of Bob Dylan’s ‘‘Like a Rolling Stone’’: ‘‘It flaps you right away.’’ The images that follow — a smoking industrial landscape in Butte, Mont.; a black nurse holding a porcelain-white baby or an unwatched black infant rolling off its blanket on the floor of a bar in South Carolina — were all different jolts of the same current. That is the miracle of great socially committed art: It addresses our sources of deepest unease, helps us to confront what we cannot organize or explain by making all of it unforgettable. ‘‘I think people like the book because it shows what people think about but don’t discuss,’’ Frank says. ‘‘It shows what’s on the edge of their mind.’’

On a trip upstate, Frank visited a Fourth of July celebration in rural Jay, N.Y., and photographed two girls in white dresses skipping beneath a huge, diaphanous American flag. ‘‘Something I really like is a big flag,’’ Frank says. ‘‘Here, people are so proud of it. In other countries you don’t feel they’re so proud of their flag.’’ Like most things with Frank, that cuts two ways. Foreign, uncompromisingly independent, Frank loathed the provincial prevarications of nostalgia. At closer inspection, the flag is torn, while along the photograph’s edge is the only visible face: a sneering boy. That the flag is transparent means that in ‘‘The Americans,’’ a reader looks, in effect, through the cloth to the image on the next page — to segregated New Orleans, where Frank made his best-known picture.

Frank passed through the city in 1955 and took a photograph of a row of passengers on a Canal Street trolley — whites in front, blacks in back. In the moment, life stilled into such clarity that Frank’s shutter needed to move only once. What he says attracted him then was something filmic: ‘‘Five people sitting, each occupying a frame.’’ But it’s a black man, his forehead creased, whose complex expression makes the picture. ‘‘He’s looked at that street many times,’’ Frank says. Plenty of photographs were taken during Jim Crow. Frank’s gift was to transcend reportage and tell you something about the condition — how oppression felt.