A handbook is a type of reference work, or other collection of instructions, that is intended to provide ready reference. The term originally applied to a small or portable book containing information useful for its owner, but the Oxford English Dictionary defines the current sense as "any book...giving information such as facts on a particular subject, guidance in some art or occupation, instructions for operating a machine, or information for tourists."
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. Examples of engineering handbooks include Perry's Chemical Engineers' Handbook, Marks Standard Handbook for Mechanical Engineers, and the CRC Handbook of Chemistry and Physics.