Google offers a special tool for searching scholarly literature. This specialized search engine covers peer-reviewed papers, theses, books, preprints, abstracts, and technical reports from many disciplines.
Some Google Scholarly literature is not peer-reviewed (evaluated by a group of experts in the appropriate field). For a definition of peer-reviewed sources and more, see The Research Process: A Guide for GWC Students.
Seek advice about the best resources by talking to your librarian.