During the golden weeks of autumn, it seemed as if everyone in the world wanted to go for a walk with William B. Helmreich. The journalist from Norway. Students who have lapped up his courses at City College and the Graduate Center of the City University of New York. The publicist at Princeton University Press, which just published “The New York Nobody Knows: Walking 6,000 Miles in the City,” his doorstop-size account of four years of trekking into every corner of the five boroughs, dead-end streets and desolate industrial areas included.

“New York is so varied,” said Mr. Helmreich, who has practically made a second career out of explaining so ambitious an undertaking. “But if you don’t walk the streets, you never really understand that. Plus my philosophy is, everything’s interesting.”

Mr. Helmreich, who is tall and blue-eyed with close-cropped gray hair, likes to call himself a flâneur, in a tip of the hat to the boulevardiers who strolled the streets of 19th-century Paris. This particular flâneur is 68, the child of parents who immigrated to New York from Switzerland in 1946 and settled in a tenement apartment on the decidedly unchic Upper West Side.

Mr. Helmreich, who also describes himself as the ultimate city kid — “I was a member of the little gang on my block” — stayed put in New York until 1984, the year that he and his wife, Helaine, a writer, moved with their three children to Long Island, albeit to a town just a 15-minute walk from the Queens border.