Look in the following works, shelved in the reference room (both Christian iconography and beyond):
An Introduction to Iconography (1994), Ref. N7565 .S7713 1994 and online .
For additional titles, Marquand's reference collection has many additional titles on more specific topics relating to subjects and symbols. One can try a Subject (browse) search in the library catalog for: Symbolism in Art Dictionaries, or consider any topic you are interested in and look for that heading in the form: X in art.
e.g. Bees in art
Also, the Index of Medieval Art is open to the public: see its website for further information about this wonderful iconographic resource and database that includes bibliography and imagery from Late Antiquity to the 16th century.
Image databases can sometimes be helpful for questions of iconography.