For the scholar, books / monographs, government documents, conference proceedings, research reports, white papers and similar documents provide a wealth of data and information and can be easily dismissed if you are limiting your research pool to peer-reviewed articles.
Most dabases index such information; and the service can be used to obtain the item in full-text (for items that we subscribe) or through ILLiad for items that we do not own.
Be aware that the service does not work well for book chapters. Instead, search the book title in Find It!
These (often unpublished) research sources can help a scholar discover methods and frameworks, as well as provide rich reference lists to aid in their own research.
While they will sometimes turn up in literature databases, there are indexes specifically for this content.
Dissertations and thesis, if not available in full-text, can often be obtained through Inter-Library Loan.