As Mr. Eyman’s detailed and at times long-winded new book, “John Wayne,” makes very clear, the man Duke Morrison — born in 1907 with the unlikely name Marion Robert Morrison in the small town of Winterset, Iowa — was not synonymous with the John Wayne character he created on screen. “In Wayne’s own mind,” Mr. Eyman writes, “he was Duke Morrison. John Wayne was to him what the Tramp was to Charlie Chaplin — a character that overlapped his own personality, but not to the point of subsuming it.”

Image John Wayne in the movie “The Big Trail,” from 1930, from a photograph included in the biography by Scott Eyman.

A conscientious and tireless student, Duke started out as an extra and prop man, and spent a decade before his breakthrough role as the Ringo Kid in John Ford’s “Stagecoach” (1939), toiling away in B pictures, made in three, six or 10 days. During those years, he honed his craft and put together the pieces of a screen character — his distinctive start-and-stop phrasing; his low, slow growl of a voice; his graceful, rolling walk. It was a character that drew upon his easy charm and faith in self-reliance, but it was also modeled, he once said, on “the kind of man I’d liked to have been.”

Mr. Eyman says the screen persona that Wayne devised for himself — of a man who was at once an outsider and an authority figure — helped transform an awkward, insecure boy into “the emphatic representation” of an “American masculinity” that was sure enough of itself to encompass a certain melancholy and vulnerability beneath the steadiness and self-assurance.

This story of John Wayne’s invention of himself has been told many times before. “The Searchers: The Making of an American Legend,” Glenn Frankel’s succinct and powerful 2013 book about John Ford’s masterwork, discussed how the young Wayne studied the western star Harry Carey’s slow, soulful manner, and the lessons he learned from the accomplished stuntman Yakima Canutt, a former rodeo star. (“I studied him for many weeks, the way he walked and talked and rode a horse and pulled a gun.”) And Mr. Wills’s 1997 book, “John Wayne’s America,” which was less a biography than an extended essay about the dynamics of celebrity, also covered much of this same ground.

Mr. Eyman’s book aims to be far more comprehensive than those earlier works; at times, it feels too comprehensive, doggedly chronicling more forgettable Wayne movies and offering tedious asides about the financing of various films. The author also spends an awful lot of space on issues that have been debated for decades (like the relationship between Wayne’s conservative politics and his choice of movies) without shedding new light on them.