Amazon.com’s Statistically Improbable Phrases, or “SIPs”, are the most distinctive phrases in the text of books in the Search Inside! program. To identify SIPs, our computers scan the text of all books in Search Inside. If they find a phrase that occurs a large number of times in a particular book relative to all Search Inside books, that phrase is a SIP in that book.
SIPs are not necessarily improbable within a particular book, but they are improbable relative to all books in Search Inside. For example, most SIPs for a book on taxes are tax related. But because we display SIPs in order of their improbability score, the first SIPs will be on tax topics that this book mentions more often than other tax books. For works of fiction, SIPs tend to be distinctive word combinations that often hint at important plot elements.
So their system identifies a bunch of phrases that seem domain-related, then mines other books for the same phrase. It’s a way to have a “Related Books” feature without human intervention.