A handbook (sometimes called a vade mecum which is Latin for go with me) is a pocket book that can be carried at all times. Now however, this term is thought to be very old and is not in normal use any more.
Handbooks may have information about any topic, and are mostly guides of information in a special field or about a special method. They are made to have fast answers in a special 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.