Though not strictly Steampunk by today's definition, many C19th writers can be considered the founding fathers of the genre.
HG Wells, Jules Verne, Mark Twain and Mary Shelley all created novels that featured advanced technologies in the Victorian era. These tales heavily influenced the boys' own adventure novels of the next forty years.
Later in the C20th, Mervyn Peake's Gormanghast trilogy and the novels of Harrison, Heinlein and Moorcock would all develop the genre before the term Steampunk was coined.