I've been reading up on the History of Middle Earth books by Christopher Tolkien, son of J. R. R. Tolkien, about the ways in which his father wrote the Lord of the Rings books, how the drafts changed over time, leading to the final, published stories.
They're very interesting, especially for budding writers, because it shows the process of change and edits and rewrites, and how next to nothing stays the same from the first writing session to what is being printed and put on bookstore shelves.
There's 12 books in the full series, I only have 5 of them, I've been reading through Book 8, I believe, The War of the Ring at the moment, very good and comprehensive.