A reference book generally is a collection of facts, i.e. it is a reference to data. Examples of reference books:
- Encyclopedias
- Dictionaries
- Who's Who
They're not the kind of book you sit down and read for pleasure or for more than a brief entry. Reference books can be found in the online collection of the Library as well as the physical collection, for example, the 
Credo Reference Collection contains all online reference books.