Dictionaries help to clarify terminology and can be a good source of potential keywords for searching.
Encyclopedias provide further detail and explanations of concepts, as well as background on key events, people and places related to the topic you are researching. Encyclopedias also provide references to additional readings for more information.
The Annual Reviews provide substantially researched articles written by recognized scholars in a wide variety of disisplines that summarize the major research literature in the field. These are often a good place to start your research and to keep informed about recent developments.