Philosophy

Librarians and scholars may use the term "reference book"differently.  Encyclopedias, dictionaries, bibliographies, and periodical indexes are traditionally placed in library reference rooms and can't be checked out so that they're available for on site consultation.  Researchers increasingly expect to find this kind of information online.  RUL often acquires online variants, but licenses are expensive. 
Books in Alex Reference can now be checked out, and older reference books may often be found in Stacks. 
The scholar's definition of "reference book" often includes authoritative monographs and standard works outside of the library definition provided above.  The Cambridge Histories series fit both groups' definition, and we have many of the component volumes in print reference collections and also online. 

Routledge Encyclopedia of Philosophy.

ALEXANDER, CAMDEN REF B51.R68 1998

Dictionary of the History of Ideas
Covers "a varied array of selected pivotal topics in intellectual history and of methods of writing about such topics" and "recounts the historical development of such ideas across a range of disciplines, including philosophy, religion, politics, literature, and the biological, physical, and social sciences."

Encyclopedia of Philosophy [Rutgers Restricted Access]
An electronic version of the 10 volume 2nd edition by Donald Borchert, Detroit: Macmillan Reference USA, 2006. Covers all aspects of Philosophy.

Internet Encyclopedia of Philosophy
A fully searchable online encyclopedia, featuring articles written by professional philosophers. Also includes a philosophy timeline and a small collection of classic philosophical texts.

Stanford Encyclopedia of Philosophy
A dynamic encyclopedia, in which each entry is maintained and kept up to date by an expert or a group of experts in the field.