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 more than 7.5 million items, but keep in mind that the library can also get you books from around the world. Try searching the collections of Ivy League libraries through Borrow Direct or libraries throughout the world using WorldCat.org.
For more targeted results, including the ability to search by subject headings, try the following article databases: