"Reference" materials are generally things that you wouldn't read the whole thing at once, just use to look up specific information. The most common reference works are encyclopedias and dictionaries. Usually, you use reference works for information, such as definitions and explanations, rather than for details of research, though they will help you understand research published elsewhere. If you use them for this purpose, most often you would not cite a reference work unless you were using a direct quotation (for example, comparing definitions from several dictionaries, or pointing out an error in an encyclopedia entry.) If you find information that you would like to use, for example, a statistical chart, see if you can find where the reference work sourced that information from. Encyclopedia entries often have good reference lists. (Consult your librarian if you have trouble or questions about this.) Other types of reference works include handbooks and guidebooks, which, as compilations of best practices, are citable.