Google is now personalizing searches, i.e., editing search results, which creates what one critic calls the "filter bubble." What this means is that the same search on Google by different people will yield different results, depending on who Google thinks is doing the searching -- and there is no way of knowing who Google thinks that is. This filtering is invisible and based on such complicated code that even Google developers could not explain search results.
One thing this means is that the old "page rank" system is no longer operative. This may have implications for Google Scholar and Google Books. For example, if I'm a 9/11 conspiracy theorist, Google will prioritize links confirming my perspective and filter out links that conflict with it.
Google's filter bubble is discussed in a few places that I've seen. There is a 52-minute program at http://www.kqed.org/a/forum/R201105191000
It's discussed here and here on Rene Pickard's blog (the first of those links providing a guess at some of the 57 factors going in to Google's filtering; the second a link to a discussion by Eli Pariser on the filter bubble.)
Edited to add: Here's a list by Eli Pariser of 10 things you can do to "pop your filter bubble.: