When James M. Cain published The Butterfly in 1946, he added a preface. Perhaps he did so because it was a novella, about half the length of a novel, and he needed to bulk up the book.
Whatever the reason, this preface gives us glimpses into his life as a writer.
He talks about his travels in Harlan County, Kentucky and how meeting people and working alongside them in the mines and elsewhere gave him the desire to write a story about them. This sounds more like immersion than research.
He says writing for newspapers prepared him to approach a subject this way, and he praises other forms of writing as useful to becoming a novelist.
He disagrees with critics who compared him to Dashiell Hammett and Ernest Hemingway, and goes on at some length to distinguish his writing from theirs.
Most surprisingly, he distances himself from "the picture business," denying accusations that he writes with adaptation for movies in mind. Coming from the author of The Postman Always Rings Twice and Double Indemnity, this is surprising, especially since several other works were adapted and he worked for a time as a screenwriter.
I've blogged about books by novelists that mix practical writing advice with memoir: Patricia Highsmith, Lawrence Block, Stephen King. I enjoy these because they teach me not only how they wrote, but what it was like to live through it. This preface by Cain is a miniature version of those books.