Google is a general search engine but with its policy of developing more and more products, in principle, free, it has been years offering two very useful for the scientific search options. One of them is Google Academic, which allows access to full text documents on many occasions, as well as to know that documents have been cited to a given document. Their big problem is that its documentary universe is unknown. Google Books, is another product of Google to take into account the scientific search. It is a search engine that allows you to search a (s) set (s) keyword (s) or phrase in the full text of thousands of books, especially current. They can not download books (some if you can read online), there are pages that are not (you have access to more if you have a Google account), but if it helps to know that texts may be of interest. There are links to bookstores to buy them.