A journal article or book is a piece of research frozen in time. It represents the state of scholarship up to about 9 - 12 months prior to publication. What you don't know is how was this scholarly article or book received? Who has agreed or disagreed with it? Who has built upon it? Who has taken the work in a totally new direction?
Citation indices will help you find the scholarship that has used this article or book since its publication.
Originally developed for the science disciplines, especially for articles, these citation indices now include social sciences and the humanities. There are some issues we'll discuss regarding citation and impact factor issues, but for now the three major resources for citation searching are listed below.