Subject headings in the library catalog (and WorldCat) are an excellent way to get at the books most relevant to you. At the bottom of each book record, you will see lists of linked subject headings. There may be many different subject headings that will be useful to you, depending on your topic, but I particularly wanted to point out that Nationalism -- [country] is a subject heading.
You can use the same searches you used effectively in the catalog (both subject and keyword), and get a lot more results! WorldCat is the closest thing to a universal catalog. It includes the holdings of thousands of libraries worldwide.
Try searching the full text of books with Google Books. You’ll get results, almost guaranteed (whether they’re relevant is another story!).