Books tend to be longer, and broader in scope, than magazine or journal articles. Therefore, the search vocabulary required for finding them may be somewhat broader in scope than that for articles. For example, while researching the bombing of the Murrah Building, you might find that books about terrorism, 3rd amendment rights, explosives, and criminal justice all have chapters or sections important to your topic. Journal, magazine, and newspaper articles, on the other hand, tend to explore narrower and more finely focused aspects of an issue or event. You may need to explore a subject over a period of time .
Terminology changes over time.
As you go back in time in your research, look for ways in which vocabulary may have changed to reflect attitude changes or a different knowledge base.