Since books vary in page size and font size, that is tricky. The only way I know of is to reduce your page size, increase your font size, or reduce your margin size, all of which would make printing sort of a pain.
What I do is just count the number of words on a page in a book and use that number as an average page. Write it down so you don't forget, then using MS Word's word-counter (Tools>Word Count) you'll take the total # of words you're written and devide it by the number you wrote down.
So say you counted 100 words from a book page, and you're written 1800 words... you wrote 18 pages. That will be an approximation, so nothing exact. Upside is that you won't have to any funky formatting.