How could The Winds of Winter get published just three months after George R.R. Martin finishes it? Over at Tor.com, there’s a great breakdown of why book production generally takes a year, and how they might streamline it for A Song of Ice and Fire.
How George’s process works:
GRRM doesn’t write drafts of books. He writes drafts of chapters (sometimes several chapters from one POV in a row), then goes back and rewrites and polishes and edits them. He the moves onto another character. Sometimes decisions made for a new character affect other, previously-”finished” chapters, leading to delays and the butterfly effect.
Once he has a whole load of chapters done that he is 100% satisfied with, anything from a couple to couple of dozen, he sends them to his editor. His editor makes suggestions which incorporates whilst still working on new material. He also writes in a non-linear fashion, moving from character to character rather than chapter to chapter (for example, he wrote almost all of Tyrion’s chapters for Book 3 as part of the Book 2 writing process). This is why he can’t simply publish whatever he has now to stay ahead of HBO, as what is 100% finished now might be Chapters 2-20, 30-33, 45-60 and he might have not even written the prologue and first chapter yet.
So when he writes the last chapter of the book and sends it to to be edited, it’s only a small chunk which needs work rather than the whole thing. A book-wide line-edit and continuity check is still required, but the in-depth editing takes place whilst the primary writing phase of the book is still underway.