I use Amazon's data feeds to come up with the calculation. Unfortunately, they don't offer word count so I've created an algorithm based on the number of pages, genre, and the dimensions of the book.
Is it accurate? Let's take a look at a few examples. The actual word count for The Hunger Games is 99,750 and my utility calculates 94,743 words. Sharp Objects has a word count of 80,565 and my utility calculates it at 80,137. Pretty impressive! Are they all that accurate? Many, yes, but there are exceptions due to the layout and font size of a particular book. For example, Atonement has a word count of 123,378 and my utility comes up with 110,765 words. So it'll be off on occasion, but it should get you relatively close for the average book.
Would you prefer a more accurate word count? I previously used the running time of the associated audiobook to do the calculation, and it was quite accurate. Unfortunately, Amazon removed that value from their data feeds. But if you?re willing to help, I can still do the calculation for you. Just access my Audiobook Conversion Utility and follow the steps.
And if you're only looking for a few examples from a particular genre, the word counts for other books listed on this site have also been verified. Word Count Ranges by Genre is a good place to start.
Once upon a time Amazon offered "text stats" for a majority of their books, which included word count. Although Amazon stopped offering this for new books several years ago, the feature still remained for older books. Unfortunately, they recently removed that feature as well.