What mean the symbols of The Twilight Saga books cover.

I have found Stephenie's comments on what exactly does the apple on the cover of Twilight means?

"The apple on the cover of Twilight represents "forbidden fruit." I used the scripture from Genesis (located just after the table of contents) because I loved the phrase "the fruit of the knowledge of good and evil." Isn't this exactly what Bella ends up with? A working knowledge of what good is, and what evil is. The nice thing about the apple is it has so many symbolic roots. You've got the apple in Snow White, one bite and you're frozen forever in a state of not-quite-death... Then you have Paris and the golden apple in Greek mythology—look how much trouble that started. Apples are quite the versatile fruit. In the end, I love the beautiful simplicity of the picture. To me it says: choice." said Stephenie Meyer.

Of course there are different theories on the web: one think that Apple means forbidden, the others believe that it's related to Adam and Eve.

The next book cover "New Moon". I found Stephenie's explanation:
"That is a ruffled tulip. As for the meaning... If you've read the Twilight FAQ, you know that the apple cover had a lot of meaning for me, and I was an active part of the covering process. However, that experience is more the exception than the rule in the publishing world. Something to keep in mind if you intend to embark on a career as a writer: lots of things you might expect to be under your control are not. Covers, for example. Those are mostly up to the publisher and the marketing and sales departments. So I don't know what the tulip means—I didn't have anything to do with this one." commented Stephenie.

So the real meaning is, that author of The Twilight Saga: New Moon didn't have any.
I believe that New Moon is more symbolized with the new life started there. As Edward left Bella she had to go through the hard times, but she had her second birth, second life with Jacob. Flower look like Indian symbol.

The cover of Eclipse pictures red ribbon teared, but still tight.
There is no much differences among theories considering this cover. Almost everyone agreed that Eclipse cover illustrates Bella's choice between Edward (her true love) and Jacob (her friend, but not just friend.)She wants to make a choice and get things right, but Bella just can't let go Jacob as he is too important.

The last book named Breaking Dawn puts everything on its places.

"Breaking Dawn's cover is a metaphor for Bella's progression throughout the entire saga. She began as the weakest (at least physically, when compared to vampires and werewolves) player on the board: the pawn. She ended as the strongest: the queen. In the end, it's Bella that brings about the win for the Cullens." explained Stephenie Meyer.