The Penguin dictionary really is a great asset, but if you start going down specific decks, you might want to get the specific books for those. The Thoth, for example, which rwcarter mentioned, has a habit of turning symbols upside down so that while their original significance is still there, it is highly modified by context. Beyond what is on the card itself, the structure and progression of a deck is in itself a symbol, and since almost all decks mimic the Golden Dawn RWS, they all have the same structure. I recommend DuQuette's Chicken Qabalah for this. It isn't a hard read, in parts it's hilarious and most importantly, it's a real eye-opener and opens any GD deck in ways unimaginable.
Apart from that, simply reading Greek and Roman myths helps a lot. The Majors each have an attribution that mirrors or complements the card in a big way, and knowing the stories adds a lot of depth. The minors have two attributions, of sorts, so there is even more to sink your teeth into. Don't worry if you don't know anything about astrology; merely knowing the stories will not only benefit you greatly and will give great insight into the cards, but they're also really good and fun to read.