Well, it obviously depends on how much of a commitment you want to make. If you're really interested in cards, I'd recommend getting Card College or The Art of Astonishment, both of which are large investments, but which both contain a smorgasbord of effects and sleights.
If you're looking for a more casual entry, I'd pick up Royal Road or something.
If you're really interested in advanced sleight of hand, grab Expert at the Card Table by S.W. Erdnase (it's free at Erdnase dot com, so you have no excuse).
That wise enough for you?