I took a look at the entry Amazon.com has for my book, and I see they’ve added some new features related to their Search Inside add-in. In addition to selected pages, they now have a concordance of the 100 most frequently used words (not surprisingly, the most common word is sharpshooter). It also has a “fog index” that is supposed to show clarity of writing (mine is 14.6, which is fairly low), and a Flesch Index (48.2, about the middle of the scale).
It also tells you the percentage of compex words (15%), the average number of words per sentence (21.6) and the total number of words (128,911). Finally, you get the number of words per dollar — 3688. Not sure how much of this goes into the decision to buy a book, but it’s fun to read.