, 15th edition: volumes of the Propedia (green), Micropedia (red), Macropedia, and 2-volume Index (blue)
reference work is a work such as a book or periodical (or its electronic equivalent) to which one can refer for information. The information is intended to be found quickly when needed. Reference works are usually  referred to for particular pieces of information, rather than read beginning to end. The writing style used in these works is informative; the authors avoid use of the first person, and emphasize facts. Many reference works are compiled by a team of contributors whose work is coordinated by one or more editors rather than by an individual author. Indices are commonly provided in many types of reference work. Updated editions are usually published as needed, in some cases annually (e.g. , Whitaker's Almanack ). Reference works include Who's Who dictionaries, encyclopedias, almanacs, atlases, bibliographies, biographical sources, catalogs such as library catalogs and art catalogs, concordances, directories such as business directories and telephone directories, discographies, filmographies, glossaries, handbooks, indices such as bibliographic indices and citation indices, manuals, research guides, thesauruses, and yearbooks. Many reference works are available in electronic form and can be obtained as  reference software, CD-ROMs, DVDs, or online through the Internet.
A reference work is useful to its users if they attribute some degree of
In contrast to books that are
loaned, a reference book or reference-only book in a library is one that may only be used in the library and may not be borrowed from the library. Many such books are reference works (in the first sense), which are, usually, used briefly or photocopied from, and therefore, do not need to be borrowed. Keeping reference books in the library assures that they will always be available for use on demand. Some reference-only books are too valuable to permit borrowers to take them out. Reference-only items may be shelved in a reference collection located separately from circulating items. Some libraries consist entirely, or to a large extent, of books which may not be borrowed.
An electronic resource is a
computer program or data that is stored electronically, which is usually found on a computer, including information that is available on the Internet. Libraries offer numerous types of electronic resources including  electronic texts such as electronic books and electronic journals, bibliographic databases, institutional repositories, websites, and software applications. 
"Reference". The Merriam-Webster.com Dictionary. Merriam-Webster Inc . Retrieved 2019.
Reitz, Joan M. (10 January 2013). "Reference book". Online Dictionary for Library and Information Science . Retrieved 2019.
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Reitz, Joan M. (10 January 2013). "Electronic resource". Online Dictionary for Library and Information Science . Retrieved 2019.
Further reading General Guides to reference works
Chenoweth, Juneal M. (ed.). American Reference Books Annual. Santa Barbara, California: Libraries Unlimited. ISBN . 978-1-4408-6913-6 Published annually beginning in 1970.
Heeks, Peggy (1968). Books of Reference for School Libraries: An Annotated List (2nd ed.). London: Library Association. ASIN B0006C36OO.
Lester, Ray, ed. (2005). . New York: Neal-Schuman Publishers. New Walford Guide to Reference Resources, Volume 1: Science, Technology, and Medicine ISBN . 978-1856044950
Lester, Ray; Clinch, Peter; Dawson, Heather; Edwards, Helen; Tarrant, Susan, eds. (2007). . London: Facet Publishing. New Walford Guide to Reference Resources, Volume 2: Social Sciences ISBN . 978-1856044981
Lester, Ray, ed. (2015). New Walford Guide to Reference Resources, Volume 3: Arts, Humanities, and General Reference. London: Facet Publishing. ISBN . 978-1856044998
Malclès, Louise Noëlle (1950). Les sources du travail bibliographique (in French). Geneva: Librairie Droz.
Sheehy, Eugene P. (1976). (9th ed.). Chicago: American Library Association. Guide to Reference Books ISBN . 978-0838902059 Originally compiled by Alice B. Kroeger for first two editions beginning in 1902. Subsequently edited by Isadore Gilbert Mudge (3rd through 6th editions) and Constance Mabel Winchell (7th and 8th editions).
Totok, Wilhelm; Weitzel, Rolf (1984-1985). Handbuch der bibliographischen Nachschlagewerke (in German) (6th ed.). Frankfurt am Main: Klostermann. CS1 maint: date format ( link) First published in 1954.
Walford, A. J., ed. (1980). (4th ed.). London: Library Association. Walford's Guide to Reference Material, Volume 1: Science and Technology
Walford, A. J.; Taylor, L. J., eds. (1987). (4th ed.). London: Library Association. Walford's Guide to Reference Material, Volume 2: Generalia, Language and Literature, The Arts Walford, A. J., ed. (1981). Walford's Concise Guide to Reference Material. London: Library Association. (an abridgement of Walford's Guide)