McLoughlin Brothers was one of the most prominent publishers of children’s publications in the world from the mid-nineteenth-century through the mid-twentieth century. They are also one of the significant American publishers during this important era in publishing history, in terms their sustained technical, printing, and marketing innovations .

McLoughlin Bros. publisher’s catalogs and marketing materials provide important research tools for those interested in the history of children’s book publishing and the business and marketing activity of a major American publisher.

Dating McLoughlin Bros. publications is notoriously difficult. Many McLoughlin books are undated; the firm also reissued some titles over and over again, often with changed inventory numbers, from one time-period to the next, keying revisions and different issue dates. Publisher’s catalogs, advertisements in the books themselves, and McLoughlin's own publisher's archive materials are often the only source for identifying publication information and for itemizing the company’s changing inventory numbers and revisions of exiting titles, as the firm constantly revised and expanded its product lines of books, games, and children’s’ toys.

The Cotsen Children’s Library holds one of the largest aggregations of McLoughlin Brothers’ catalogs, many from the firm’s own business office and publisher’s archive, uniquely annotated by the publisher with information about new editions, changes in illustrations, or new series. In addition, Cotsen holds ten large, elephant-folio-sized, scrap-book-style “guard-books” containing proof copies of illustrations for books, many extensively hand-annotated by McLoughlin Brothers' editorial or production staff with information about the original appearance in print or repurposing of illustrations in later editions. These guard-books also include the firm's notations about the physical location of the blocks and plates used to print the illustrations, and were used as an index or internal "finding aid" to the blocks and their precise locations.