"Grey literature" refers to stuff that is rigorous and well-researched, but doesn't neatly fit into the major categories of peer-reviewed articles and scholarly books.
Examples of grey literature:
Whether or not grey literature will be useful and appropriate depends on your information need and where the information is coming from.
A comprehensive database and search engine for public policy grey literature (information published outside of traditional commercial or academic publishing channels), including reports, policy briefs, working papers, and other materials.