Using quotations around standard phrases, such as "Civil war" can narrow your search to relevant results.
When you find a book that is on-topic, take a look at the subject headings assigned to the book; you can use them to identify other relevant titles. Subject headings can be general, or very specific. Some examples of Library of Congress subject headings include:
Use reference lists/bibliographies to locate other research materials (chaining): locate a book that has potential for your topic or interests. Scan the reference list to look at the sources used for the author's research; do title keyword searches in the library catalog to locate the article, dissertation, or book.
Browse the stacks or the catalog. Again, find a book of interest and go to the library stacks to locate it. Because classification systems (either Library of Congress or Dewey) assign call numbers by the subject of the book, you'll find other items in the same general area.