Robert Donald's talk, THE TRUTH ABOUT APPLES, explores the symbolism of fruit in art. Often, scant attention is given to the significance of what might be regarded as incidental and small details in painting and sculpture, such as fruit, flowers or casual domestic objects.   However, such details in both early and recent works can elaborate and give fuller meaning to the subject and intentions of the artist. 

In addition to web research into the general and specific subject of symbolism, the following reference books will be of help:
Sarah Carr-Comm The Dictionary of Symbols in Western Art New York: Facts On File, 1995
George Ferguson Signs and Symbols in Christian Art New York: OUP USA, 1977
Michael Baxendall Painting and Experience in 15th Century Italy Oxford: Oxford Paperbacks, 1988
Erwin Panofsky Meaning in the Visual Arts London: Penguin Books, 1993

General reading of good books on specific artists will often help to 'break the code'. Another area to delve into is that of Classical mythological personalities and storylines which have their own areas of symbolsm. Once the beginnings are mastered, it becomes a fascinating topic to follow up on art of all periods, but especially early art. Eventually, the obsession can lead to an Agatha Christie mentality as you search for that last clue! Be warned!