The book "Introduction to Information Retrieval"

It turns out that there is a great book on the search technology (Information Retrieval) from Cambridge University Press. Html and pdf versions are freely available. Work on it is still underway, or recently completed (last updated April 7, 2009). The book is written in excellent English, there are good exercises. In general, I recommend to all who are interested in how Google and Yandex work!

Html version - nlp.stanford.edu/IR-book/html/htmledition/irbook.html
PDF-version (very beautiful, I recommend!) - nlp.stanford.edu/IR-book/pdf/irbookonlinereading.pdf

But about the Information Retrieval in Wikipedia - en.wikipedia.org/wiki/Information_retrieval