Tertiary sources compile, summarize, index, or help identify scholarly sources on a topic. They are useful for background research or to get an overview of a topic before delving into secondary or primary sources. Common tertiary sources for Chemistry and Biochemistry include dictionaries, handbooks, encyclopedias, and other reference sources.
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A dictionary or encyclopedia can be a useful starting point if you are unfamiliar with a concept. The resources can give you the terminology that might be useful when searching. Some dictionaries give very brief explanations of key words; others give a paragraph. An encyclopedia focused on a particular subject is often arranged alphabetically, with several pages of information and background, plus lists of resources for more information. They are written by experts in the field.
In addition, handbooks are very useful in research. They often summarize important concepts, and are used by students and practitioners alike.
Best bet for short definitions - Hawley's Condensed Chemical Dictionary. Best bet for CAS Registry number: Merck Index.
Hawley's Condensed Chemical Dictionary by For very detailed, lengthy articles, with lists of references, try first Kiirk-Othmer Encyclopedia of Chemical Technology.
For detailed, and less lengthy articles, consider Ullmann's Encyclopedia of Industrial Chemistry.
For brief information, and comparison tables for chemicals and substances, try first Lange's Handbook of Chemistry.
Kirk-Othmer Encyclopedia of Chemical Technology by 
Ullmann's Encyclopedia of Industrial Chemistry
Lange's Handbook of Chemistry by 
Encyclopedia of Biological Chemistry by JoVE focuses on teaching the practice and theory of scientific experiments - from videos. It has clear, detailed illustrations of experiments.
