I've got another question from a reader (and if you have a writing question you'd like me to tackle, let me know!).
How long should a book be, and who decides this, the author or the publisher?
There's a practical side and an artistic side to this.
On the practical side, there are some publishers who have specific target word counts they're looking for. This is especially common in category romance (Harlequin and Silhouette), where books are part of a line and are packaged within certain parameters. If a publisher has a length requirement, it will generally be in the publisher's guidelines.
For adult genre fiction, the "standard" length is about 80,000 to 100,000 words (I don't know what the standards are for young adult or children's books, but they're generally shorter). These days, they're edging closer to the lower end of that range. That doesn't mean that all books fall within that range or that they must fall within that range, but that's how they base a lot of their planning (costs, shelf space, how many books fit in a box for shipping). If you're a new author, you may have better luck staying within that range -- or, at least, not going way over it. The reason is simple economics. Longer books are more expensive to produce. They use more paper and ink, the covers have to be bigger to wrap around the fatter spine, they're more expensive to ship, and they take up more shelf space, which may mean fewer books on the shelf, which means fewer sales opportunities. There are a couple of ways publishers can avoid a fat book being less profitable, and neither of them are good for new authors. They can raise the cover price -- and do you really want your book to be more expensive than the other books like it? Or they can shrink the margins and type size and make the book smaller, which can turn off a lot of readers. It's kind of like in screenwriting where they encourage new writers to come up with scripts that will be cheap to produce (small cast, few locations, few special effects or stunts). If a company is having to take a financial risk on an unknown, they'll be more comfortable with a smaller risk.
Of course, if you've written an absolutely brilliant book that has "bestseller" written all over it, they may not balk at a 200,000-word manuscript. But books that fall within the standard range will probably have a better chance of being published, especially in today's economic climate. "Name" authors have more leeway, but right now, I'm trying to keep things in the 90,000 word range, myself. Even after you've been established as an author and aren't trying to sell your first book, they may ask you to trim a book. They're not going to tell JK Rowling to cut 20,000 words, but on my last two books they asked me to try to trim about 10,000 words when I was doing editorial revisions.
On the artistic side of things, I like the way one of my journalism professors said you should judge the best length of a news article: It's like a bikini -- it should be big enough to cover the important things and small enough to be interesting. A book should be as short as it can be and still tell the story. It should use as few words as possible to tell that story. That doesn't mean you have to cram an epic fantasy into 80,000 words, but it does mean that you'll probably have a better book if you can trim a 125,000 word manuscript to 90,000 words.
That's because when you do that kind of cutting, you're eliminating anything that's not essential, and the result is that you shouldn't have any boring or unnecessary parts where the action slows. You'll have eliminated redundancies in wording and in scenes. You'll also probably find any place where one particularly powerful word can do the work of many. All of that adds up to a book that's a real page-turner and that will require fewer resources to publish it, which makes it even more attractive to publishers.