The FIRST answer to that one, of course, is another question: "WHICH book?"
Because here it is folks, one of the fundamental truths of the craft.
There are several different kinds of writers.
One, the writer who tends to write the same SORT of book again and again. I suspect (although I have never written one, so those who have can feel free to chime in and correct me if I am wrong) that writing things like category romances tends to be a fairly regimented process because of the fairly rigid set of rules which governs the finished product. In other words, if you're writing to a pattern or an expectation and you're on your third, tenth, twenty seventh book, you've got it honed down to a fine art. You can probably churn out a decent story within a month - you're putting together a product very similar to something you've done before and familiarity breeds speed and confidence.
Two, the writer of a fabulously successful series whom the market is driving to writing more books in that series whether (s)he wants to or not - and boy, how that shows sometimes - and here it's something similar but not exactly identical to the above process, because you're not necessarily dealing with the same general PATTERN but you do have to take into account a certain amount of immovable stuff which you have to write around - things you've already established in previous books, a certain setting, a certain character, stuff you can't just arbitrarily shift because you've now made it canon and the reader will roar in outrage if you futz with canon.
Three, the rest of us. And for the rest of us... and this is truth... even if the fundamentals of the writing process which we find most comfortable remains basically unaltered... EVERY BOOK IS DIFFERENT.
Here's a selection of paths by which I arrived at my own works. See if you can match process to finished book (I'll put the answers underneath, under a cut).
1) Wrote a single scene featuring the protagonist and a handful of the main characters - a scene which is a definite high point in the storyline - liked the scene, and set out to write the book in which it appeared. But when I started writing the story, and I began to write it fairly linearly, from the beginning, it took me literally 2/3 of the tale to actually GET TO THE FRIGGING SCENE WHICH STARTED IT ALL. Really, you would never know - and if I asked you to pick the scene I am talking about, from that story, all y'all would probably pick a different scene. Truth is, it's integral to the plot, to the book, and it is impossible, once the story was done, to actually pry that one single brick out of the mortared wall. It is impossible to tell that the entire wall once hinged on the existence of that one single brick, or which brick it was - but once the brickness was established the wall was built relatively mundanely, one further brick at a time. The whole effort took... years. At least a year to write, and then more years before it saw publication.
2) Wrote down a list of ten characters. Nameless, milieu-free characters. Just a short paragraph about each. My husband asked me what that was and I told him it was my next novel - but I had NO clue what the story was that these characters wanted to tell. At this point somebody sent me a newspaper article about a real-life situation - and the fantasy which involved that news story and those characters blossomed into my mind, fully formed, with the characters taking on a vivid and brilliant life and literally dictating the book to me (I wrote 200,000 words in LESS THAN THREE MONTHS). I didn't stop to think, to breathe, practically not to sleep or eat - I wrote it at white-hot fury. What's more the draft I wrote down was not draft zero or even draft one. It was pretty much it, the finished thing, with a few tweaks but no major changes. In *200,000 words*. It was a miracle book.
3)Was asked, after finishing the miracle book, if there was a sequel. I denied it, right until the moment... there was one. An editor was involved with this one right from the start; we discussed the bones of the book, I presented a loose sack of ideas, she approved them, and I wrote the book. It came back to me with an editorial fiat that unequivocally demanded that I rewrite the ending completely. I did. It still worked. You probably couldn't tell that there was ever a different ending to this thing than the one it currently bears.
4) A combination of a series of ideas culled out of frustrations with the popular culture, a real-life but rather larger-than-life character I wanted to write about, and a desire to explore a different magic gelled to produce a story about a youngster coming into her potential through some fairly fraught circumstances. It was a difficult, in many ways, story to write, because it was more structured than some of my other tales were - and I don't work well to outlines at all, at all. In fact, looking back over the proposal that this series was sold on, it bears so little resemblance to the synopsis that I originally sent in that my OWN jaw drops when I compare what should have been with what actually emerged as the finished product. Once again, reading the books that were actually written, you'd never know that there was ever a discrepancy - it fits, it works, it came together. But this story took me longer to write than anything I had written before. In pure wordage, it adds up to no THAT much more than book @2, above - but that took me less than three months, and by the time I was done with all of #4 it was closer to two years all told. Still. It hangs together.
5) I posted on Facebook recently that I had just had a nice 3000-word writing day. Which was entirely true. But the project about which I was speaking has been percolating for some considerable time. It started life as a potential short story which I began as a possible contribution to a themed anthology which was knocking about, oh, maybe a year ago. I kind of realised at one point that the story I was writing had the hallmarks of a very good story indeed - it had a nice "voice", a good character as a protag, an intriguing premise - but that I was almost 5000 words into it *and I was still worldbuilding*. This was no short story. It wanted to be a novel. But this was a novel which now presented me with a myriad of locked doors - and I spent long months unlocking them, one by one, hoping to find a way into the entrance hall but instead finding myself blundering about in the kitchen or feeling my way out of a dark closet or wondering how in hell I had got into the bathroom or sitting in an empty parlour with the blinds drawn and I couldn't see out. I had all these little bits, see, and I KNEW they were more or less parts of the same edifice but I just couldn't seem to find the right way inside it in a manner that would let me make sense of it. Until those 3000 words which I squeed about on Facebook - because here we were at last, in the entrance hall which had eluded me so long, and yes indeed there WAS a welcome mat on the floor. But here's where something interesting happened. Yes, I had written 3000 new words and I had discovered my beginning - but all those other jigsaw puzzle pieces began to make a strange sort of sense. So I went back to the bathroom, the kitchen, the closet, and discovered I actually DID have a map of an entire house. And if I fitted certain things together in a certain way... I had a considerable amount of story I didn't know I had, as it were. So, then. Less than 48 hours ago I posted a "new" word count of 3000 words. Today I have a chunk of novel that is 17,000 words long. Almost literally overnight, I'm heading towards a fifth of a good-sized novel.
You want to know how I write a book?
WHICH BOOK?!? They are all different for me. Every. Single. One. I reinvent myself as a writer with every single manuscript I produce. And I guess the one single answer to that vexed question is that there IS no single way that a book can be written or has to be written. If it works for you, and produces something good, it all comes out even in the end.
The answer is not to obsess if you're writing a book "the right way". There IS no right way. No two books are exactly alike. Listen to them, and they'll tell you what their preferred process is. And after that... just TRUST them. Your stories know what they are doing.
1) Hidden Queen/Changer of Days
2) Secrets of Jin Shei
3) Embers of Heaven
4) Worldweavers trilogy - "Gift of the Unmage", "Spellspam", "Cybermage"
(most of the above also available for your Kindle...)
5) Current Project. No, I won't say any more about it right now. But watch this space.