Definitions for "Reference Book"
A book designed by the arrangement and treatment of its subject matter to be consulted for specific information or facts, rather than to be read consecutively. Reference books cannot be checked out of the library, but you are welcome to use them while in the building. Sometime older editions of reference books are available in the stacks.
a book designed to be consulted for specific items of information, as a yearbook, atlas, or dictionary.
Materials such as encyclopedias, periodical indexes, bibliographies, handbooks, and directories.  Since these materials are frequently used they cannot be checkout out.