Reference work

A reference work is a work, such as a book or periodical (or their electronic equivalents), to which one can refer for information. 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 use of the first person, and emphasize facts.
Indices are a common navigation feature in many types of reference works. 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. Updated editions are usually published as needed, in some cases annually ("Whitaker's Almanack", "Who's Who"). 
Reference works include 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.
Reference book.
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.
Types of reference work.
These are the main types and categories of reference work:
Electronic resources.
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.