Just everything that's ever been written about Tolkien and his life and work. Including at least 3 biographies that I know of, countless scholarly papers dedica

Just everything that's ever been written about Tolkien and his life and work. Including at least 3 biographies that I know of, countless scholarly papers dedicated to his linguistic work as an academic and fantasy writer, literally volumes of material that have been published by his son, himself describing his creative process, etc. It's a pretty well-known thing that he was a linguist first, wrote the hobbit to read his children as a story that fit in with the languages and mythology he'd already developed, and only ever considered publishing them because his friends (including C.S. Lewis) badgered him about it.