A reference work is a compendium of information, usually of a specific type, compiled in a book for ease of reference. That is, the information is intended to be quickly found 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. Indexes are commonly provided in many types of reference work. Updated editions are published as needed, in some cases annually. Typical reference works include dictionaries, encyclopedias, and compendiums (see further examples below). Many reference works are available in electronic form and can be obtained as software packages or online through the Internet.

In comparison, a reference book or reference-only book in a library is one that may only be used in the library and not borrowed from the library. Many such books are reference works (in the first sense) which are usually used only briefly or photocopied from, and therefore do not need to be borrowed. Keeping them in the library assures that they will always be available for use on demand. Other reference-only books are ones that 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 or individual reference-only items may be shelved among items available for borrowing.