Boy is this a moving target now with the rise of the ebook revolution.
However, its best to break it down by genre - in PRINT Books:
Overall for Adult books:
80,000 – 89,999 words
SCI-FI AND FANTASY
32,000 – 40,000 words
55,000 – 69,999 words
NOW with the rise of the ebook all of the above is different if you DON'T PLAN ON PRINTING the book.
With short story formats in ebooks rising many authors don't need to hit the higher word counts above for adult books in order to make the reader happy.
WHY THE STRINGENT cap on word count?
Well as authors we love to get lost in the story, but for a publisher to PRINT a book and EDIT a book the cost to benefit rises by page/word count. Therefore the Publisher has to pay more to print the books which means the PROFIT hurts with the larger word novels if the publishers wants to COMPETITIVELY PRICE the book for sale with other books in the genre.