I'll second GeorgiePorgie's recommendation. If you are to get one general, comprehensive book on symbols, I'd say this is it! It incorporates (and corrects) much of Cirlot (the best book before the Penguin book came out).
I got ARAS's The Book of Symbols. While there are some interesting bits, on the whole I was rather disappointed in it—especially as regards tarot.
In addition to the books mentioned by Scion, I also find myself referring to:
• Funk & Wagnall's Standard Dictionary of Folklore, Mythology and Legend
• Dictionary of Symbolism by Hans Biedermann
And, if you're interested in a Jungian psychological perspective, then
• Ariadne's Clue: A Guide to the Symbols of Humankind by Anthony Stevens.
Also, for the RWS deck, you might want to look at works on symbolism in Freemasonry, as well as alchemy, medieval number symbolism, etc.
There are lots of other books that will add essential bits and pieces, but, for a single book - I'd go with the Penguin Dictionary of Symbols.