Reference works summarize the physical and chemical properties of substances. These works cross-reference substances by various names, registry number or other assigned identifier and reaction conditions for quick look up access. There will be references back to the original work. The key is they summarize know verifiable information, hence they are 2° literature.
Below is a listing of dictionaries, encyclopedias, handbooks & atlases, treatises & reviews, biographical resources and directories resources in print and electronic format.