"Reference materials" typically refer to encyclopedias, dictionaries, bibliographies, atlases, maps and other related works. They can be very useful to the researcher. At the same time, they tend to serve a very specific, limited function in the research process. To cite a few examples:
Source: Bolner, Myrtle S. The Research Process: Books and Beyond. Dubuque, Iowa : Kendall/Hunt Pub. co., 2001.