"Reference sources" can provide helpful background information about a topic and include sources such as encyclopedias, topic overviews, dictionaries, timelines, and bibliographies.
Online resources such as Wikipedia can be a great start, but usually will not be considered reliable. The following ebooks and databases have background information from credible sources which you can use and cite in your academic assignments.
A Dictionary of Nursing by 
Encyclopedia of Nursing Research by 
The Gale Encyclopedia of Nursing and Allied Health by 
Dictionary of Nursing Theory and Research by Search over 600 full text reference books for your topic. It also has article links to library databases (such as ProQuest and EBSCO).
Includes language and subject dictionaries, general reference books, subject-specific reference books, timelines, quotations, and overview pages.
Information on health topics, drug information, and a medical dictionary.