I'm thinking it might start with their algorithm for finding improbable phrases. They serve as a kind of a word-based fingerprint for a book and probably make searching on over-specific queries like this one a lot easier.

http://www.amazon.com/gp/search-inside/sipshelp.html

An easy way do it would be to extract the term vectors from the query and then find related documents computing the cosine similarity measure.

Not too long ago Amazon did not return anything if one word was wrong. They had "The Chocolate Bible : The Difinitive Sourcebook" and would not find it in a search with "definitive" spelled correctly. They have a better search now.

Well my guess would be that they have a structured data back end of titles, authors, and so forth. They can then look at the query string and extract everything that looks like a proper noun (or doesn't exist in a dictionary) and and compare that against their structured data database.

I'm sure it's more advanced than that but that would get you pretty far.