On http://www.amazon.com that is - the co.uk version doesn't seem to do this.

Apparently they have searched inside the text of the books and then compared oft-repeated phrases with phrases that appear regularly in all the books in their catalogue.

For a given book they then highlight the phrases that don't appear in general in all the other fiction books. Hence, for Hard Boiled Wonderland and the End of the World we find:

Statistically Improbable Phrases (SIPs):
unicorn skull, chubby granddaughter, shuffled data, gas inspector, chubby girl, core consciousness, old officer

I guess it's some sort of internal searching mechanism, but I don't really see why they bother to show us on the page for that item?