When the George M. Hill Company, the original publisher of The Wonderful Wizard of Oz (1900), went into receivership in February 1902, two of its employees, production manager Frank Kennicott Reilly and head salesman Sumner Charles Britton, decided set up a firm of their own. They quickly formed the Madison Book Company to handle the technical books of Hill's list. Soon they expanded their plan, to encompass new work as well. Needing a name author for their venture, they courted Baum, who was unhappy with his existing publisher Bobbs-Merrill. On 16 January 1904 the new partners signed Baum to an exclusive contract; starting with The Marvelous Land of Oz, Reilly & Britton was Baum's publisher for the remainder of his career.
The firm published no less than six Baum books in 1906, and five in 1907; there followed three per year in 1908 and 1909, four in 1910 and 1912, and five in 1911. Thereafter Reilly & Britton issued two or three Baum books per year until Baum's death in 1919. The firm looked out for its star author: when Baum became embroiled in financial difficulties in 1909 and after, Reilly & Britton paid him a monthly stipend instead of the annual royalty checks that were standard in the industry. (The arrangement, however, was not enough to prevent Baum's bankruptcy in 1911). Baum was personally close with both partners, though more with Britton than Reilly; his personal correspondence to them was addressed to "Brit" and to "Mr. Reilly" respectively.
Though Baum was their best seller, the company published a range of other material. They were strong on children's books, publishing classics like Hans Christian Andersen and the Brothers Grimm, and popular and topical series books too, like The Airship Boys and The Boy Scouts of the Air. The firm issued abundant sports-related materials, especially baseball books; it published popular authors like the poet Edgar Guest (best known for "The Gingham Dog and the Calico Cat"). The publishers' judgment was not infallible: they turned down a chance to print Edgar Rice Burroughs's first Tarzan book.
The firm's structure changed in 1919: Britton sold his partnership to longtime employee William F. Lee. The first edition of The Tin Woodman of Oz (1918) bore the Reilly & Britton imprint, and all subsequent editions, Reilly & Lee. After the death of L. Frank Baum, Reilly & Lee appointed Ruth Plumly Thompson to continue the Oz series. Reilly & Lee conducted some imaginative publicity for its Oz books in the 1920s, with contests, a fan club, plays for children, and other tactics — though these largely stopped with the death of Frank Reilly in 1932.
The company never became anything more than a middle-level publishing enterprise, and its fortunes faded by mid-century as interest in the Oz books waned. Frank J. O'Donnell served as president of the firm in the 1940s and into the 1950s, when Jack Snow and Rachel Cosgrove were the "Royal Historians" of Oz.
When Baum's original Wonderful Wizard of Oz passed out of copyright protection and into the public domain in 1956, Reilly & Lee issued its first edition of the book, with illustrations by Dale Ulrey.
The Henry Regnery Company bought Reilly & Lee in 1959, ending its independent existence. Regnery later assigned the rights to Oz to Contemporary Books, which in turn was absorbed by McGraw-Hill. (Anecdotes, hard to verify, maintain that Henry Regnery personnel cleaned out Reilly & Lee files after purchasing the company — and that Dick Martin rescued original Oz art from the Regnery dumpsters.)