The cards are messages between your (well, you can call it lots of things...higher self, guide, angel, unconcious, superconcious, etc etc...it's the same thing...) and your concious, "ego" self.
You higher self (or whatever) can track whatever meanings you happen to retain and assign to a given card. It will gradually "stretch" you to see more and more meanings and those can get added to the meanings you already have, or re-assigned/re-programmed by you, your concious self...it's as if you say, "Ok, higher self, this card is going to mean THIS to me. And it will. Untill you (or your higher self) decides to change it. When that happens, you'll be sitting there, and the card will come up, and you'll say to yourself, "hmm, I know this card usually means X, but today it seems to mean Y." Then, you have a card that meand X and Y. And don't worry about memorizing it all, your unconcious/higher self is in charge of all that, and will remind you, and bring to the fore, the meaning it wants you to apply today. Personally, I think it's good to read books (cause that's the way I did it, and of course, I did it the right way ;-). Read bunches and bunches of them if you want... and if you see something that really leaps out at you and gives you one of those "OF COURSE! EUREKA!" moments, then great! If not, don't worry, you'll forget it along with the rest of the stuff you've forgotten. But the stuff that really impresses you does so because it touches deep, and your higher self will somehow (who knows how, really) retain that, and remind you.