That question just arrived in my email, and it's a good one. The answer is: It depends.
Some publishers, especially in genres like romance, have requirements as uniform as a muffin tin: Each manuscript will be long enough to fill a book of exactly so many pages. Others are far more flexible.
In the days of the typewriter, a double-spaced page with 1-inch margins would hold an average of 250 words. So you could assume that since 4 pages = 1000 words, 240 pages = 60,000 words, which was the typical length for most mainstream and mystery novels.
Then came the personal computer and the writer's own choice of typeface and point size.
This threw the old estimate right out the window, but word processors of course give us a precise word count—and word count is what most publishers are interested in.
The number of words in a novel will vary depending on the genre. A young-adult novel will run between 20,000 and 40,000 words. That would be the length of a novella in mainstream fiction.
Many genres seem to have succumbed to supersizing. A mass-market western, mystery or SF novel used to run to 60,000-70,000 words. Now they're more likely to hit 90,000 words. Fantasy novels seem to be about 125,000 words—per volume, with some series going on and on. Mainstream fiction can be anywhere from 55,000 words (about the length of many mass-market romances) to ten times that.
And how many pages will your published book have? Again, book design and typesetting can influence the total. I've noticed a lot of hardback mainstream fiction lately with text that's really double-spaced, with generous margins: clearly the author handed in a short manuscript, and the publisher is trying to puff it up into something that looks worth the price.
Mass-market paperbacks don't seem to have changed their design much, however, and I've noticed that a typical page of a paperback novel will run around 440 words...not quite the equivalent of two double-spaced manuscript pages. My novel Eyas was just about exactly 500 pages long in manuscript, and came out in print at 354 pages.
If you're working in a genre like SF or romance, it's always a good idea to visit the websites of genre publishers to see what length they want. If you go to Harlequin and browse through their FAQs, you'll see that different romance series require much different lengths, from 50,000 words to twice that.
For you this is a craft, maybe even an art. For them, it's an assembly-line business. If you're J. K. Rowling or Stephen King, you can tell your publishers how long your next novel will be, and they will tug their corporate forelock, thank you very much, and plan accordingly. But lowly scribes like you and me are unlikely to enjoy such freedom.