The John Mystery books are a collection of Australian children's books and ephemera produced by a little known publishing dynamo, Lester Sinclair, in the middle of the twentieth century. I identify factors which operated to position these items as forgotten elements of Australian literary history. After contextualizing the John Mystery brand of children's books, I suggest how children's literature scholars may find potential resources in the Children's Literature Collection and other heritage collections of the State Library of Victoria (Author abstract).