Words per page depends on publisher, book format, and period, rather than genre.
For instance, during WW2, paper rationing was severe, and in the UK paperbacks used thinner and cheaper paper, smaller type, and narrower margins, to squeeze a book onto less paper. In addition, peace time books would usually end a chapter, leave the rest of that page blank, and start a new chapter on a new page, sometimes a quarter or third of the way down the page. This wouldn't happen in the wartime; there wouldjust be a line or two skipped between chapters. A book that might be, say, 230 pages in hardback before the War could have become 180 pages in wartime paperback.
Also, because paperbacks were for many years purely budget productions, they were deliberately kept compact, again using smaller type and narrower margins.
Then there's the writer's style. Erle Stanley Gardner's Perry Mason books contain a great deal of dialogue in short question and answer form, during long trial scenes for instance, which leaves a lot of white space on the page (ie fewer words per page). Writers who prefer long narrative sequences and longer speeches can leave very little white space.
This is why page count isn't a lot of help in determining the size of a book, because so much depends on publishing houses' styles. It's why print publishers and writers prefer word count.
|