Chevalier and Gheebrant's Dictionary of symbols (Penguin paperback), any day. Not only it is an invaluable ressource for tarot major arcana (all 22 major arcana are explained), it is also a great ressource for symbols generally, including many you will find even on non-standard tarot decks; for numbers, for alchemy, etc etc. Got a snake in a Lovers card (Ancestral Path) - find out all about snakes and sex! Got a phoenix, a seed, a dog, a singing mermaid? (well, not sure about the last one, but a mermaid certainly) - they're all there.
I've had it for years, and it's still a pleasure beyond compare.