Take a look at subject headings for ideas to extend or narrow your search. Put phrases in quotes, and use truncation (an asterisk * in the Catalog) to search for alternative endings and spellings of words (e.g., iraq terror* will pick up terror, terrorism, terrorist, etc., while labo*r Britain politics will get results for both labor and labour).
Princeton owns millions of items, but keep in mind that the library can also get you books from around the world. You can search the collections of Ivy League libraries through Borrow Direct or libraries throughout the world using WorldCat.org.
Searching Articles+ is a great way to get directly to the full text of journal articles and ebooks we have access to. For even easier access (especially when you're off-campus), install the Library's new tool:
It provides one-click access to journal articles from scholarly and publisher websites, Wikipedia, PubMed, and more.