Leavy covers her biographical bases: She revisits Ruth’s upbringing at a Catholic reformatory school in Baltimore, the hasty and unhappy marriage he made in Boston and his sullen retirement spent largely in exile from baseball. But it’s this “new kind of love” that illuminates her portrait of Ruth. Her aim is to prove “how thoroughly modern he was, not just in the way he attacked a baseball, but also in the creation, manipulation and exploitation of his public image.”

The barnstorming tour provides a colorful backdrop for this side of the Babe Ruth story. Ruth, however, can only properly be called its co-author. The barnstorming trip was the handiwork of Christy Walsh, Ruth’s agent avant la lettre, a kind of Scott Boras in spats. For long stretches of “The Big Fella,” Walsh takes center stage. He was the impresario behind Ruth’s syndicated newspaper columns, which were ghostwritten by a cohort of sportswriters (several of whom also covered Ruth for their papers — a cozy arrangement for the Babe). He negotiated many of Ruth’s endorsement deals. And he closely guarded Ruth’s reputation — from those members of the press not in his pocket and from Ruth himself, whose insatiable appetites for food, drink and women caused Walsh fits.

Walsh had a keen eye for what we now call “synergy,” and in this, at least, he had an obliging client. “There was no frame he couldn’t or wouldn’t fill,” Leavy writes of Ruth. “No pose he wouldn’t assume. No one he wouldn’t pose with. Posing was the only time he stood still.” In one entertaining chapter, she recounts the barnstorming stop in Omaha, home to 1927’s other great record breaker: a hen who had laid an egg on 171 consecutive days. The press had christened her the Babe Ruth of egg-laying. A meeting of the two Babes would provide free publicity for the remaining dates on the tour — and reinforce the image of Ruth as a wholesome man who could be trusted in the henhouse. “One a day for 171 days!” the Babe exclaimed when he met the bird. “Gosh, how I wish I could do as well!”

Elsewhere, however, Leavy can strain to find meaning in the marketing materials. It’s true, and worthy of note, that Ruth’s celebrity was so novel that it outstripped the capacity of American law to protect it. This allowed men like Otto Y. Schnering, the manufacturer of the Baby Ruth candy bar, to profit from Ruth’s name without compensating him. (Schnering claimed, with comical disingenuousness, that his product was named for Grover Cleveland’s daughter, who had died of diphtheria 15 years before the first Baby Ruth arrived in stores.) But Leavy’s long detour into jurisprudential debates over publicity rights, and Ruth’s failed effort to popularize his own Home Run bar, will try the patience of readers who lack a strong taste for legal or confectionary history.

For a manifestly assiduous reporter and researcher, Leavy can also be careless with the facts. Describing Ruth’s endorsement of Benrus wristwatches, she introduces quartz technology several decades too soon. Comparing Ruth’s pitchman prowess to that of O. J. Simpson, she has Simpson the running back shilling for Avis, not Hertz. Minor errors, but they undermine the reader’s confidence that Leavy has separated the man from the myth.

And few ballplayers have been as mythologized as Ruth. In one well-worn tale recounted by Leavy, Ruth spends a day in a Manhattan lockup, having been caught speeding around the city in his maroon torpedo roadster. A gaggle of reporters swarm the courthouse, hoping to catch a glimpse of the Bambino behind bars. “I see a shadow!” says one enterprising photographer, who has climbed a fire escape for a better view into Ruth’s cell. “Snap the shadow!” replies a reporter on the street below.

Ultimately, this is what Leavy has done in “The Big Fella” — snapped the shadow. The book captures Ruth’s outsize influence on American sport and culture, and for that alone it will make a welcome companion during the long, baseball-less months to come. But the man of many poses never fully comes into focus.