A second way to get a lot out of Google is to perform a topic or author search in Google "Books." A search for an author, for example, Martha Crenshaw, a well known terrorism researcher, will bring up books by Crenshaw and also books that mention Crenshaw.
The truly terrific thing about Google books is that many have parts duplicated on the web. I would NOT recommend reading a little bit of a book as if it were the whole--go to the library and get the entire book. But the fact is that running down a list of books and reading bits of them is about as close as you can get in cyberspace to the nice experience of walking down a bookstore or library and browsing related books just to see what's there. You may pick up new resources, surprising facts, or just what you were looking for and didn't know by "browsing" this way. Don't forget to check out indexes and bibliographies for even more sources!