If you search for products a lot, using Google, there's a fairly good chance you've seen Amazon at or near the top of the results pages quite a few times. Someone sent a question about this to Google, and Matt Cutts used a Webmaster Help video to discuss the subject.
The question was phrased as: "Search for a physical product usually ranks Amazon #1, even though it may not provide the best user experience. What is being done to prevent large corporations from dominating search engine results?"
Matt's responded by saying, "I think in general, not to call anybody out, but I think Amazon does have a relatively good user experience in general. I wouldn't necessarily agree that Amazon always ranks number one for every physical product."
"So typically when I do a search for a book, Amazon is up there, but if there is an official homepage for a book, it often ranks very well, and sometimes number one as well," he continued. "The interesting thing is not every book has a home page. This is something that still surprises me. You'll have a very savvy author. They'll have a webpage, but they may not have a landing page or a page dedicated to that specific book. Sometimes it's just a lack of savviness."
He brought up one book that he had recently looked at, noting that no other content about it was on the web, other than Amazon, GoodReads, and Google eBooks.
"The best answer is, make sure there is an actual page for your product," said Cutts. "In general, Google does try to figure out what are the official home pages whether it be for governments, universities, or states or whatever, and we try to make sure we return those when possible."
"We are mindful of whenever users do a search, and then they complain to us, if they complain that they're not finding an official homepage for a product, then that's something that we do take into consideration," he said. "In general, we do look at the number of links. We look at the content of the page, and if one particular website is able to get a lot of links, because a lot of people thank it's a great site, then in general, usually it should rank relatively well, and I think that by itself isn't necessarily a problem."
Looking at this a little bit myself, I did find that a search for my wife's book, "The Fireman's Daughter" did return an Amazon result within the first few results (a band with the same name is ranking above it), while the landing page for the book from the actual publisher is buried 5 pages in. There are pros and cons to Amazon outranking this page. She makes more in royalties if the book is purchased directly through the publisher, but on the other hand, the Amazon brand also lends a bit of trust from the user's perspective, as not as many people will be familiar with the publisher itself (this may be a different story with some more well-known publishers).
The question is not just about books though. Looking at it from the perspective of the average online store, the consumer trust factor likely plays a big role in Amazon's rankings. Remember Google's list of questions you could use to assess the quality of your site? It included something like "Would you feel comfortable giving your credit card information to this site?"