Various forms of encyclopedias can be a good first introduction to a subject. Both encyclopedias and dictionaries provide basic definitions of terms. Here you can find examples of important concepts within a discipline and can use them as search words when doing a literature search in databases. There are both general and subject-specific encyclopedias. Sometimes articles and entries in reference books and encyclopedias contain a list of references for further reading.
Check out Britannica online which includes the complete encyclopedia, as well as Merriam-Webster's Collegiate Dictionary and the Britannica Book of the Year.
Down below you will find examples of encyclopedias and dictionaries relevant to your subject areas.