(** = Highly recommended)
"Gray Literature" is literature (often scientific or technical) that is not available through the usual bibliographic sources such as databases or indexes. It can be both in print and, increasingly, electronic formats.
Gray literature is produced by government agencies, universities, corporations, research centers, associations and societies, and professional organizations.
Gray literature is an important source of information. Though not scholarly, it is produced by researchers and practitioners in the field. It can often be produced more quickly, have greater flexibility, and be more detailed than other types of literature. "Gray literature serves scholars and lay readers alike with research summaries, facts, statistics, and other data that offer a more comprehensive view of the topic of interest (Weintraub)."
In the future, gray literature will be even more important. "In a world in which free trade and instantaneous communication have eliminated many of the barriers to information flow, grey literature is gaining greater importance as a source of information for much of the world's population (Weintraub)."
For more information about gray literature, try IL Toolkit-Finding Information: Gray literature.
Gray literature, due to its diverse origins and unpublished nature, can be difficult to find. Gray literature is often found by searching for the agency or institution who is most likely to produce the literature. The search may require looking at a large number of sources. The World Wide Web has made the dissemination of gray literature easier and advances will continue in the future.
Here are a few places to start looking for gray literature: