According to Joan M. Reitz's Online Dictionary of Library and Information Science, a reference book is a ...

A book designed to be consulted when authoritative information is needed, rather than read cover to cover. Reference books often consist of a series of signed or unsigned "entries" listed alphabetically under headwords or headings, or in some other arrangement (classified, numeric, etc.). The category includes almanacs, atlases, bibliographies, biographical sources, catalogs, concordances, dictionaries, directories, discographies and filmographies, encyclopedias, glossaries, handbooks, indexes, manuals, research guides, union lists, yearbooks, etc., whether published commercially or as government documents. Long reference works may be issued in multivolume sets, with any indexes in the last volume... In libraries, reference books are shelved in a separate section called the reference stacks...