Tutt Library Research Guides
You can take a single source and "explode" it and find other related sources backward, forward, and parallel in time.
To go backward in time, look at the bibliography or notes in the source itself. That will show you previous works that were used in building the source.
To go forward in time, you can use a search engine that has a cited reference feature. The most accessible is Google Scholar, which usually displays this feature as "cited by" X number of other sources. That will show you works that were published after the source which point back to it in their notes and bibliography. Other databases will also show how other sources in that particular database cite the source in question (Historical Abstracts and JSTOR are examples). Web of Knowledge is the granddaddy of citation searches, and is usually the most complete, but is also a bit more difficult to use.
For parallel sources (or sources where time isn't a consideration), use keywords or subject headings drawn from the source itself or the citation of the source in the database.