Library of Congress Subject Headings is an alternative to keyword searching. They can narrow down your search much more effectively than keywords.
Here are some National Security subject headings to use in your searches:
JFL's InterLibrary Loan service can get books and other media from other libraries for you to use in your research. If we don't have what you're looking for here, ask ILL to find it.
US Grand Strategy in the 21st Century by 
Terrorism and Homeland Security by 
A Practical Introduction to Homeland Security and Emergency Management: From Home to Abroad by 
Encyclopedia of United States National Security by 
Foreign Policy Begins at Home by 
Homeland Security Intelligence by 
