Davidson, Sara. “A Cycle of Revolving Research.” University of California Libraries. http://library.ucmerced.edu/rpt/knowledge_cycle_8.
Generally, the category of works we call "reference" includes almanacs, bibliographies, dictionaries, directories, encyclopedias, guidebooks, handbooks, indexes, manuals, yearbooks—materials we "refer" to rather than read entirely. Go to these for definitions, formulas, codes, standards, and topic overviews or background information.