NUSearch, Northwestern University Library's search tool, can be used to find books and other library resources. To search for books, it is best to click on the "Books, Images, & More" tab; otherwise, you are likely to be overwhelmed with articles.
"ninth amendment" OR "9th amendment"
slavery AND constitution
equal protection AND same sex marriage
or if you want to get really sophisticated: ("ninth amendment" OR "9th amendment") AND (privacy OR sex*)
It's also possible to click on "browse search" and browse titles, authors, or subject heading strings. Some subject headings that may be useful:
Representative government and representation -- United States
United States. Constitution 9th Amendment
United States. Constitution 14th Amendment, etc.
Firearms -- Law and legislation -- United States
Eminent domain -- United States