Literature reviews are a particularly useful resource when doing research. They address the issues involved in a particular question and the debates among scholars. They map out the intellectual terrain succinctly and give you the major landmarks in terms of key authors and significant titles for greater understanding.
Usually researchers find more sources by looking at the footnotes in an article or book, but these will always be older than the publication you have in hand.
Citation indexes like the Web of Science (which includes sections for the Social Sciences, Arts and Humanities, and Science) are set up to search for sources cited in the footnotes of journal articles as soon as they become available. This allows you to find newer articles which cite the books and articles you already know are key for your topic. By relying on connections between authors rather than subject words and by moving forward in time, citation searching can open up new avenues of research.