The book "Introduction to Information Retrieval"

It turns out that there is an excellent book on search technologies (Information Retrieval) from Cambridge University Press. Html and pdf versions are freely available. Work on it is either ongoing or recently completed (last updated April 7, 2009). The book is written in excellent English, there are good exercises. In general, I recommend to everyone who is interested in how Google and Yandex are arranged!

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

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