A handbook is a type of reference work, or other collection of instructions, that is intended to provide ready reference.
A handbook is sometimes referred to as a vade mecum (Latin, "go with me") or pocket reference that is intended to be carried at all times. Also it can be referred to as an enchiridion.
Handbooks may deal with any topic, and are generally compendiums of information in a particular field or about a particular technique. They are designed to be easily consulted and provide quick answers in a certain area. For example, the MLA Handbook for Writers of Research Papers is a reference for how to cite works in MLA style, among other things.
"Handbook" is sometimes applied to documents that are produced within an organization that are not designed for publication—such as a company handbook for HR, for instance. In this case, the term is used nearly synonymously with "manual."
The name "handbook" may sometimes be applied to reference works that are not pocket-sized, but do provide ready reference, as is the case with several engineering handbooks such as Perry's Chemical Engineers' Handbook, Marks Standard Handbook for Mechanical Engineers, and the CRC Handbook of Chemistry and Physics. Handbooks are widely used in the sciences as quick references for various kinds of data.