EVERYTHING!
That’s not a PR answer, either. I love the struggle to find a plot that really works for the characters emerging from my imagination. I love the creation of prose that tells their tale their way. I love the follow-up detail work, buffing the words, spackling in tighter structure, sanding down the pacing. I love the unplanned twinkles, when parts of a book resonate with each other all without my planning it. I love the research, from figuring out which words are anachronistic, to digging into entire biographies to mine a single, priceless detail of the period.
I love it all, and I’m happy when I’m working on a book. When a book can also brighten a reader’s day, then I have joy to go with my happiness, and that just makes me want to write MORE.