Dorothy Seymour Mills, who helped her husband, Harold Seymour, compile the first in-depth historical studies of baseball but who did not receive full credit for her research, editing and writing until after his death, died Nov. 17 at a hospital in Tucson. She was 91.

The cause was complications from an ulcer, said Jacob Pomrenke, director of editorial content of the Society for American Baseball Research.

In the 1940s, Harold Seymour was working on his doctoral dissertation in history at Cornell University, working on the 19th-century origins of baseball. At the time, he was teaching in Cleveland at Fenn College, which later merged with Cleveland State University. Ms. Mills was one of his students.

They struck up a relationship and, after his divorce, were married in 1949. She helped with his research as he received his doctorate in 1956, then revised his dissertation for a book, “Baseball: The Early Years,” published in 1960. It was the first time the subject had been addressed by a trained academic historian and became a landmark in the study of baseball.

A second volume, “Baseball: The Golden Age,” appeared in 1971, followed by “Baseball: The People’s Game,” in 1990. Seymour was lauded as a groundbreaking historian who examined baseball as a business enterprise and cultural phenomenon of far-reaching social importance.

“For three decades Harold Seymour has been not merely the most respected of the game’s historians but also the standard-setter against whose work all others have been weighed,” Washington Post book critic Jonathan Yardley wrote in 1990. “Seymour accomplished what no one before him had: He legitimized baseball as a subject for serious historical inquiry, and in the process he accumulated more information about the game than had been available in any other published histories.”