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The DSM is the most widely used diagnostic tool in the United States for mental disorders. Used by practitioners and scholars in fields like psychology, social work and medicine. It provides the criteria to diagnose a patient with a specific mental disorder. The current edition is the DSM-5 published in May 2013. (Accessed through Psychiatry Online.)
This set covers all aspects of psychology including biographies with a focus on both historical overviews and current and future trends. See the Synoptic Outline of Contents in v. 8, p. 353, for places to start.
Look here for practical and authoritative psychology information: assessment and diagnosis; testing; psychotherapy and treatment; working with groups, families, and youth; ethical and legal issues; and much more.
Find entries on important people, movements, events, and more in this massive set covering all areas of the social sciences (psychology, sociology, etc.). The Thematic Outline in v. 1, p. CXXIII, can help you start exploring.