In this context, handbooks are defined as publications that provide theoretical overviews rather than practical instructions. Handbooks contain collections of chapters which cover a certain subject area or field. They can provide you with an introduction, the theories that are used within the research, and its most important authors, as well as good reference lists of the most important contributions to the field.
Handbooks explain the various terms used within the specific field. Therefore, handbooks provide a shortcut to important original sources.
Note: the above content is directly quoted from the University Library Guides
offered by the University Library at Jönköping University