A reference work is a work, such as a paper, book or periodical (or their electronic equivalents), to which one can refer for information.^[1] The information is intended to be found quickly when needed. Such 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 opinions and the use of the first person, and emphasize facts.

Indices are a common navigation feature in many types of reference works. Many reference works are put together by a team of contributors whose work is coordinated by one or more editors, rather than by an individual author. Updated editions are usually published as needed, in some cases annually (Whitaker's Almanack, Who's Who).

Reference works include textbooks, almanacs, atlases, bibliographies, biographical sources, catalogs such as library catalogs and art catalogs, concordances, dictionaries, directories such as business directories and telephone directories, discographies, encyclopedias, filmographies, gazetteers, glossaries, handbooks, indices such as bibliographic indices and citation indices, manuals, research guides, thesauruses, and yearbooks.^[2] Many reference works are available in electronic form and can be obtained as reference software, CD-ROMs, DVDs, or online through the Internet. Wikipedia, an online encyclopedia, is both the largest and the most-read reference work in history.^[3]

In contrast to books that are loaned, a reference book or reference-only book in a library 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.^{[citation needed]} 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.

These are the main types and categories of reference work:

• Abstracting journal – a published summary of articles, theses, reviews, conference proceedings etc. arranged systematically
• Almanac – an annual publication, listing a set of current, general or specific information about one or multiple subjects
• Annals – concise historical record in which events are arranged chronologically
• Atlas – a collection of maps traditionally been bound into book form
• Bibliography – a systematic list of books and other works such as journal articles on a given subject or which satisfy particular criteria
• Biographical dictionary – an encyclopedic dictionary limited to biographical information
• Books of Quotations – collections of quotations satisfying particular criteria, arranged systematically
• Chronicle/Chronology – a historical account of events arranged in chronological order
• Compendium – a concise collection of information pertaining to a body of knowledge
• Concordance – an alphabetical list of the principal words used in a book or body of work
• Dictionary – a list of words from one or more languages, systematically arranged and giving meanings, etymologies etc.
• Digest – a summary of information on a particular subject
• Directory – a systematically arranged list of names, addresses, products, etc.
  • Business directory
  • Telephone directory
  • Web directory
• Encyclopaedia – a compendium providing summaries of knowledge either from all branches or from a particular field or discipline
• Gazetteer – a geographical dictionary or directory used to provide systematic access to a map or atlas
• Glossary – an alphabetical list of terms in a particular domain of knowledge with the definitions for those terms
• Handbook – a small or portable book intended to provide ready reference
• Index – a publication giving systematic access to a body of knowledge
• Lexicon – a synonym for a dictionary or encyclopaedic dictionary
• List – a published enumeration of a set of items
• Manual – a handbook providing instructions in the use of a particular product
• Phrase book – a collection of ready-made phrases, arranged systematically, usually for a foreign language together with a translation
• Ready reckoner – a printed book or table containing pre-calculated values
• Thematic catalogue – an index used to identify musical compositions through the citation of the opening notes
• Textbook – a reference work containing information about a subject
• Thesaurus – a reference work for finding synonyms and sometimes antonyms of words
• Timetable – a published list of schedules giving times for transportation or other events
• Yearbook – a compendium containing events relating to a specific year

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.^[4] 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.^[4]

• Higgens, Gavin, ed. (1984). Printed Reference Material. Handbooks on Library Practice (2nd revised ed.). London: Library Association. ISBN 978-0853659952.
• Katz, William A. (2001). Introduction to Reference Work, Volume 1: Basic Information Services (8th ed.). New York: McGraw-Hill. ISBN 978-0072441079.
• Katz, William A. (2001). Introduction to Reference Work, Volume 2: Reference Services and Reference Processes (8th ed.). New York: McGraw-Hill. ISBN 978-0072441437.
• Lynch, Jack (2016). You Could Look It Up: The Reference Shelf From Ancient Babylon to Wikipedia. New York: Bloomsbury Press. ISBN 978-0802777522.

• Chenoweth, Juneal M., ed. (24 June 2019). 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 Walford Guide to Reference Resources, Volume 1: Science, Technology, and Medicine. New York: Neal-Schuman Publishers. ISBN 978-1856044950.
• Lester, Ray; Clinch, Peter; Dawson, Heather; Edwards, Helen; Tarrant, Susan, eds. (2007). New Walford Guide to Reference Resources, Volume 2: Social Sciences. London: Facet Publishing. 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). Guide to Reference Books (9th ed.). Chicago: American Library Association. 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. First published in 1954.
• Walford, A. J., ed. (1980). Walford's Guide to Reference Material, Volume 1: Science and Technology (4th ed.). London: Library Association. ISBN 9780853656111.
• Walford, A. J.; Taylor, L. J., eds. (1987). Walford's Guide to Reference Material, Volume 2: Generalia, Language and Literature, The Arts (4th ed.). London: Library Association.
• Walford, A. J., ed. (1981). Walford's Concise Guide to Reference Material. London: Library Association. (an abridgement of Walford's Guide)

• Media related to Reference works at Wikimedia Commons