It’s a lot more complex than it seems.

When I’m designing a book now, the most important thing I have to do is make castoff. This means figuring out a way to fit enough words on a page so that the book comes out to the number of pages that were budgeted for. The page count is determined very early on, by people who’re more concerned with profit and loss than with beauty, and it’s intimately tied to the price of the book and the projected sales. Some books need to be stretched so the publisher can justify charging a certain price; many, many more need
to the crammed so that they don’t cost more to produce than sales are likely to recoup. I do not have any input into this decision-making process. I just receive a stack of manuscript and a standardized worksheet showing how the page count was estimated. This worksheet shows not only the total number of pages but also the number of characters per page that will be needed to hit said length.

Because of the way printing is done, pages ideally come in groups—signatures—of thirty-two. Theoretically you can tack a sixteen-, eight- or four-page signature onto the end of a book, but it costs more to print 8 pages than to print 32, so many publishers won’t allow it. So a book might have 304 or 320 or 352 or 384 pages, but not 316. And most publishers avoid leaving more than four blank pages in the back, because blanks make the buyer think he’s being cheated. There are certain pages in the front of a book
that can be added, reshuffled, or deleted in a pinch, but the goal for the designer is to make everything fit just so without reorganizing anything.

Interesting stuff.