Since you’ve never asked, I thought I’d talk about how I write a book. 🙂
Building blocks.

Gotten build with something
I had a steep learning curve writing my first book. My training and experience is writing scripts. Dialogue heavy, light on description, character and story focused with a page limit. When it came to the first book, at first I was overwhelmed by the space I was suddenly afforded. I could go deep. I could go wide. I could go anywhere…
OK. I wasn’t going to go anywhere. I had a story and I had a genre, so there was always a destination.
The process was helped along in that the first book was based on a TV spec I had written about a year before. The script was essentially the treatment for the book. Things changed — characters transformed, a whole plot line got dropped — but it was the same basic mystery.
A script and a book are different, but they each built in blocks.
The primary building block of a script is scenes. And then, in Network TV, those scenes are grouped into acts. Each act leading to a cliffhanger or twist that makes you want to come back after the commercial break. Books, generally, are built around chapters or chapter like things, discrete chunks of story.
So, I could keep the same basic shape as the script while also expanding and deepening the story. Then came the second book…
There’s Not A Plan…

Totally not me.
I’m not a planner. I have a tendency to throw myself into something and figure it out as I go. (I’m very comfortable doing a lot of rewriting). I have always been this way as a writer, taking an initial image or line of dialogue or what the crime of the book will be and seeing where it lead me. But it took me twenty some years of writing to be able to say: this is my process, this is how I write, I need a little something-something to pull me through the project.
I used to improvise. Friends and I would get onstage, get a suggestion and go, creating scenes for twenty or so minutes. It was thrilling to not know what was going to happen but to trust that listening and being honest in the moment would lead somewhere. And that’s how I write. That little bit of terror. That touch of thrill. And to be honest, there are days it isn’t fun and it’s terrible. But then there are days that I’m surprised and overjoyed when I write something I could never have planned.
However, that’s no way to write a book. At least, for me. There has to be some shape. The chapters need to build toward something.
And that’s when it happened, while I was working on my second book…
36 Chapters.

beginning of a new scene…
I hit upon the idea of starting out with 36 chapters. Just 36 empty chapters. Placeholders, really. This would be the general shape of the book. And I chose 36 for two specific reasons. The first book was just a hair over 30 chapters, so it seemed a reasonable length and 36 is divisible by a lot of numbers.
2, 3, 4, 6, 9, 12 and 18, to be specific.
Because I’m not planning out the whole thing, I can think about the book in chunks, much like acts in a script. As I’m writing, I know I’m heading toward something, a twist, a big moment, or something where the stakes get raised. These moments are pivot points where the story can change direction.
So, if I’m thinking in 3s, I might looking at landing the clarity of the protagonist needs to do at the end of chapter 3. Then as I’m writing I’m looking out for twists or events that could happen around Chapter 6 or 9 (Probably 9 at the beginning because faster twists make for faster books. In other words, I’d start narrowing the the space between twists later toward the end of the book.)
I think about this quote a lot:
When in doubt have a man come through a door with a gun in his hand. — Raymond Chandler

Raymond stares deeply into your very soul…
Something needs to happen at this pivot points. Something surprising, something compelling the reader to turn the page. I don’t need to know the whole story, so long as I’m looking for these moments, these opportunities to surprise myself — and the reader.
In the end, the book might not end up being exactly 36 chapters. Some chapters in the initial draft might be too short to warrant being a chapter. Or maybe a chapter gets cut because I find a better way to express the moment. (Or it gets cut because it doesn’t matter.)
So there you go. That’s how I start. Now, 36 chapters, that’s sort of a random number. Not totally, it’s divisible. The real power is in the grouping. As I write without a plan, I don’t have to worry about the arc of the whole, I just think about the arc within the arc of the whole. That might be 4 chapters. Might be 6. This way, the marathon of writing a book doesn’t feel overwhelming. There’s always an end in sight.