Medium Matters: Photography in the Age of Mechanical Reproduction discusses various photographic mediums used in books from the 19th century. It covers salted paper prints, cyanotypes, albumen prints, carbon prints, Woodburytypes, and platinum prints. Each process is described in terms of its history, technology, identification, and preservation. The document aims to educate readers on the various photographic mediums found in books and why studying these processes is important.