Toby Lester was an editor at The Atlantic when a news release crossed his desk announcing the Library of Congress’s purchase of a map. It had been created by one Martin Waldseemuller in 1507. The land mass across the Atlantic was being defined with every new voyage of discovery, and Waldseemuller gave it a name: America. The map, bought for a record $10 million, is considered to be “America’s birth certificate.”

A “captivating little story,” thought Lester, whose penchant for arcane subjects had already led him to write about alphabet change in Azerbaijan, what the music of ancient Greece sounded like and unexpected harmonies found in machines we use every day. At the University of Southern Maine’s Osher Map Library, where he gave the annual Edmund F. and Virginia B. Ball lecture in December, Lester described how the “little story” about how America got its name grew into “The Fourth Part of the World.”

Confronted by “a constantly shifting mosaic of geography and history, people and places, stories and ideas, truth and fiction,” Lester turned the anecdote into the saga of how “Europeans gradually shook off long-held ideas about the world, rapidly expanded their geographical and intellectual horizons to arrive at a new understanding of the world as a whole.”

Not since I read Jared Diamond’s “Guns, Germs and Steel” have I been so completely captivated by a book.

It begins with the history of the Waldseemuller map itself, or rather the latter-day search for it, from publication to its exhibition, “encased in the most sophisticated and expensive picture frame ever made” at the Library of Congress. It is an intriguing tale whose ins and outs provide a mirror for the author’s larger quest into the map’s earlier origins.

Lester’s survey of late-medieval to early-Renaissance thinking is a tour de force. European history in all its aspects — political, religious, economic, etc. — is brought to bear on how the study of geography was envisioned and transformed over the course of three centuries. Breathtaking in its sweep, it is equally engaging for its style.

Medieval maps were supposed to interpret doctrine as much as geography. Lester also points out the implication behind the words of one chronicler: “I desire and wish that what the ear hears, the eyes may see.” At a time when few could read, the contents of these volumes — to produce any one of which might require “the skins of a whole flock of sheep” — were read aloud. As for including anything more than the simplest diagrams: “Most scribes just didn’t do complicated illustrations, much less cartography.”

One unlikely spark for a new view of the world came at a Council of the Church intended to resolve the claims of three competing popes. “Salvation itself could scarcely save this gathering,” reported one participant; but, Lester writes, “Enforced idleness had unexpected benefits. Not since antiquity had so many learned figures from so many regions of the world (been) thrust alongside one another in meeting halls and churches and eateries and lodging houses, with plenty of time to kill.” The result was “a vigorous exchange of ideas — and manuscripts.”

Besides the map makers and explorers themselves, Lester’s dramatis personae include kings and khans, princes and dukes, friars and cardinals. Some are famous, some not, such as the horse trader turned spy who, at the risk of his life, smuggled out of Portugal the seamen’s charts vital to Waldseemuller’s work.

However, the three linchpins of Lester’s tale are Columbus, Vespucci and the second-century Greek astronomer Claudius Ptolemy, who first used longitude and latitude as a mapping tool. His seminal “Geography” was well-known to scholars in the Greek-speaking half of the Christian church, and Lester conjectures that had it “made it to the West in the early 1300s, the history of cartography and indeed the European Age of Discovery might well have unfolded very differently.”

But, unfortunately for the West, “the Latins and Greeks weren’t getting along” in the early 14th century. Its reintroduction had to wait for another council — one that tried, and failed, to heal the rift between the Catholic and Orthodox churches.

As to who deserved to be the New World’s namesake, Christopher Columbus or Amerigo Vespucci, battle was joined almost as soon as Waldseemuller’s map was published. “All things considered, it was an epic fuss — quite something for an obscure little treatise on cosmography to have stirred up.” The Spanish refused to refer to “America” for two centuries.

Lester is judicious about taking sides in the dispute. Explorers in the second half of the 15th century were bedeviled quite as much by what they knew as by what they didn’t. Columbus could not shake his belief that he had arrived in Asia; the islands he found could so easily be identified on the old maps of the East.

Lester makes the case that “maddening, unknowable Vespucci,” sailing down the coast of South America, made the discovery that allowed Europeans to recognize that they had found not a new route to a known continent, but a new world, one that Waldseemuller called America.

Freelance writer Thomas Urquhart is the former director of Maine Audubon and author of “For the Beauty of the Earth.”