In the fall of 1940, while the world was waging a mechanized war, a gray-headed poet sat down to write a book about his boyhood in the Sangamon Valley.

Before her death in 1939, Skinner drew up a list of rivers the series would cover. The Hudson, the James, the Ohio, the Susquehanna, the Wabash, the Arkansas, the Columbia–the list rolled westward, but it rolled right past the Sangamon. The only mention Skinner made of it was at the bottom of a memo: “Illinois R. will include such small streams as Spoon and Sangamon.”

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The Sangamon may have been only a stream in geographical terms–it can’t even make it to the Mississippi on its own, relying on the Illinois to carry its waters there–but it has a story to tell that rivals those of greater rivers. When Abraham Lincoln came to Illinois in 1830, he settled on a farm overlooking the river, and never lived away from its banks until he went to Washington. The Sangamon flows past New Salem, where Lincoln was postmaster, and also right through Springfield.

Masters knew he had a historian’s task, but he felt free to impose his own sentiments and prejudices on the story of central Illinois.

“Memory is a kind of reading glass under which spots of earth long beloved take on the aspect of something magical, as of a miniature world examined with godlike eyes,” he wrote, acknowledging his own nostalgia.

When the series petered out with the publication of The Yukon in 1968, it comprised 64 titles. Two in particular are still considered classics. The Everglades: River of Grass (1947), by Florida newspaperwoman Marjory Stoneman Douglas, stands as a landmark of American environmental writing, and inspired the foundation of the conservation group Friends of the Everglades. Great River: The Rio Grande in North American History, by southwestern novelist Paul Horgan, won both a Pulitzer and a Bancroft Prize for American history in 1955. (Although commissioned and written for the series, Horgan’s book does not technically belong to it, owing to Farrar & Rinehart’s last-minute decision to publish Great River as part of the firm’s 25th-anniversary list instead.)

“He had lived much more triumphantly than he knew,” Gray wrote of Cook. “For by destroying in Illinois the inclination to make slaves of men, he had prepared it as a testing ground where, later, it could be dealt with as a national issue. Illinois, having been through the struggle once, knowing all the arguments and all the answers, being neither self-righteously smug in its denunciations of slavery nor inclined to hug it hysterically to its heart, was the ideal and inevitable place to become a forum of freedom.”