In this context, handbooks are defined as theoretical overviews. Within subject areas of technology, handbooks could also refer to manuals or practical instructions.
Handbooks contain a collection of overview chapters that highlight different aspects of a subject. They will give you an introduction to a specific field, 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.
Below is a selection of handbooks that may be relevant in your subject areas.