Sources for the scholarly literature review must come from from scholars and relevant experts in the subject.  Typically, such sources are peer-reviewed and come from accepted journals recognized in the field of study for the topic of the literature review.

Distinguishing between Popular, Trade and Scholarly Publications
(Time: 4:48)

OWL:  Social Work Literature Review Guidelines

University of California-Santa Cruz Library

University of North Carolina-Chapel Hill Writing Center

University of Wisconsin-Madison Writing Center

An annotated example of a literature review may be found at:

http://faculty.mwsu.edu/psychology/Laura.Spiller/Experimental/sample_apa_style_litreview.pdf

An example of a published, peer-reviewed literature review:

Allen, R.C. (1996).  Socioeconomic Conditions and Property Crime:  A Comprehensive Review and Test of the Professional Literature.  The American Journal of Economics and Sociology, 55, 293.

A good literature review should be as comprehensive as necessary to identify all of the major works and debates on your research subject. Here are some tips for going beyond basic keyword searching in order to find as many sources as you can:

• Subject-specific Databases - search in databases specific to your discipline of study to find more sources in your field. For example, EconLit specializes in economics, and will have more coverage of the economic literature than an interdisciplinary, all-purpose database such as Academic Search Complete. You should also search in more than one database/catalog since no one search tool covers everything.
  

• Subject Headings - also called descriptors, these terms are assigned to items to describe their content, or what they are about. Subject headings often facilitate more precise searching as they eliminate the need to search multiple phrases and synonyms for the same concept. Look for subject headings on items in the library catalog and in databases of journal articles. Many databases also provide a thesaurus, or index, of the subject headings used.
  

• Author Search - many researchers will write about the same topic for their entire career. Searching by an author's name may garner additional relevant information.
  

• Bibliography Mining - use the list of works cited from a relevant source to locate additional related sources.  This is a way to look for relevant sources published prior to the one in hand.

• Cited Reference Searching- search for items that have cited a relevant source. This is a way to look for relevant sources publised since the item in hand was published. 
  

Texas A&M University Writing Center:  The Literature Review (46:07)

University of Maryland:  University College:  Writing the Literature Review Pt I  (5:22)

University of Maryland:  University College:  Writing the Literature Review:  Pt. II (7:40)