William Nack, a longtime sportswriter whose deep grasp of his subjects and stylish prose invigorated a biography of Secretariat that many consider to be one of the greatest racing books ever written, died on Friday, according to Sports Illustrated, which employed him for more than two decades. Nack was 77. Nack was Sports Illustrated’s lead turf writer from 1978 to 2001, and his accounts of the sport ranged from the heroic to the tragic. While best known for his ambitious reports centering on the sport’s equine superstars, he was also unflinching when recounting racing’s worst moments, including the horrific breakdown of Go for Wand in 1990 and racing’s chronic troubles with drug abuse. “Secretariat: The Making of a Champion” was Nack’s first book, published in 1974, one year after Secretariat won the Triple Crown. The book was an intensely researched and reported biography of the horse, as well as an intimate portrayal of the human beings surrounding him, all told in a vibrant prose that earned the book a hallowed spot on the bookshelf of nearly every serious racing fan. “A book that is the next-best thing to watching Secretariat run,” the legendary sportswriter Red Smith wrote. Nack won seven Eclipse Awards, six for Sports Illustrated. In 1992, he was voted the Walter Haight Award winner by the National Turf Writers Association, honoring lifetime achievement. Last year, Nack received the PEN/ESPN Lifetime Achievement Award for Literary Sports Writing. Nack was born in Chicago on Feb. 4, 1941, when the city was a horse-racing giant. As a child, Nack groomed horses in nearby stables, and he traced his early passion for racing to his visits as a 14-year-old to Washington Park to see Swaps run on several occasions. In high school, he worked on the backside of Arlington Park as a groom for Bill Molter, the trainer at the time of Round Table, who would be enshrined in racing’s Hall of Fame. After a two-year stint in Vietnam, where he worked as an assistant editor and a public-relations specialist for Gen. William C. Westmoreland, Nack took a job with Newsday on Long Island, where he covered politics for four years before his editor wisely moved him to the sports desk to cover racing. It was while at Newsday that Nack did most of his reporting for the Secretariat book. Sports Illustrated came calling in 1978, four years after it had excerpted “Secretariat” in a widely read article. In addition to racing, Nack most notably covered boxing for the magazine, but he often strayed toward other subjects that caught his eye, including in-depth profiles of legends in their fields. In 2001, he left Sports Illustrated to freelance. Socially, Nack loved to tell stories, and he often regaled audiences with meticulously crafted narratives of his work to report out various pieces he wrote. In addition, Nack enjoyed literary parlor tricks, and his favorite was almost certainly a gravel-voiced, somber recitation of the last several paragraphs of “The Great Gatsby,” a book that Nack considered one of the finest works in the American pantheon. In addition to his biography of Secretariat, Nack wrote an autobiographical account of his career called “My Turf” and a meditation-like biography in 2007 of the star-crossed filly Ruffian that was subtitled “A Racetrack Romance.” In 1990, Nack returned to his favorite subject to eulogize Secretariat for a piece in Sports Illustrated called “Pure Heart.” The piece was named to the magazine’s “Most Memorable Stories” list in 2014. “For me,” Nack wrote in the piece, describing Secretariat being led into a stall at Claiborne Stud to begin his stud career, “that final walk beneath a grove of trees, with the colt slanting like a buck through the autumn gloaming, brought to a melancholy close the richest, grandest, damnedest, most exhilarating time of the my life.” Nack is survived by his wife, Carolyne, and four children.