The cover of Twilight has an apple. The apple is a biblical representation of the knowledge of good and evil. When Eve ate the forbidden apple in the Garden of Eden, she became aware of the presence of good and evil. Bella symbolically eats the apple and becomes aware of the existance of vampires, both the good ones (the Cullens) and the evil ones (James).
The cover of New Moon has a tulip losing a petal. This represents the loss that Bella suffers when Edward abandons her. The falling petal can also symbolize the tears she cried and the drop of blood from the paper cut that prompted Edward's decision to leave.
The cover of Eclipse has a ribbon that is almost (but not quite) ripped apart. This represents Bella's difficultly in severing her ties with the human world. Notice that the ribbon is almost cut but not quite. Bella is almost ready to leave her human life behind, but there are some attachments that she is not ready to let go (Jacob, family, sex).
The cover of Breaking Dawn has two chess pieces: a pawn in the background and a queen in the foreground. In chess, the pawn is the least significant piece, while the queen is the most significant (and most powerful). Bella is both chess pieces. She begins the book as the pawn, weak and easily overcome. She transforms into the queen, the most powerful force to stand against the Volturi.
This information can be found in The Twilight Saga: The Official Illustrated Guide. Incidentally, the cover of this book shows puzzle pieces, symbolizes the coming together of all four books from the saga.
Meyer, Stephanie. The Twilight Saga: The Official Illustrated Edition. New York, New York: Little, Brown and Company, 2011. 539.
By dropping an apple, Bella acts out another symbolic story, because she imitates the story of how Isaac Newton discovered gravity by watching a falling apple. Gravity is treated as "forbidden knowledge" associated with "the fall" in some respects by people who agree with Darwin's theory of evolution but also believe in anthropogenic global warming. This is because, in order to overcome gravity and lift water and carbon dioxide into the air, heat must constantly be supplied to the earth's oceans. When water and carbon dioxide are evaporated or otherwise lifted out of the ocean, that consumes energy that would otherwise be available for heating. When they form clouds in the atmosphere, that shuts out sunlight that would otherwise heat the earth, so water and carbon dioxide naturally cool the earth overall, offsetting the effects of the sun and gravity that would otherwise make the earth warmer. Gravity, as the unmentioned warming agent, is therefore treated as forbidden knowledge by some people.
Bella's personal environment changes dramatically when she moves from Phoenix to Forks; she might be said to undergo a "climate change" caused by an overall increase in water vapor (a "greenhouse gas") because she moves from a hot dry desert to a cool humid rainforest. However, it's a climate change that's forbidden knowledge to people who believe in anthropogenic global warming because her environment becomes cooler, wetter, and greener, rather than hotter and browner. She moves from the first city in the world where a manmade elevated carbon dioxide level is believed to have been detected to a sparsely inhabited place where levels of "greenhouse gases" (primarily water) are naturally much higher because of much greater humidity.
Sunlight, gravity and forbidden knowledge are also associated with each other in Twilight because sunlight is conspicuously absent from the pale skin and dark cover of the book; it can be imagined only as something that would have been used to grow the apple. When Edward overcomes gravity by carrying Bella above the level of the clouds (in the movie), he gives the forbidden knowledge of his true nature to Bella when the sun shines on him. Therefore, he lifts something against gravity (first an apple symbolizing Bella, then Bella herself) and offers Bella forbidden knowledge, both when he flips the apple up with his foot and when he carries her up the mountain. In both cases, he also emphasizes to Bella how dangerous he is, even as he becomes more attractive to her. This is a repetition of the Biblical quote that precedes the Twilight novel, which essentially says, "eat this fruit, and you'll die."
The themes I've discussed here appear prominently in the anthology Evolve 2: Vampire Stories of the Future Undead:
I also discuss them at the links referenced below.
The Twilight apple represents several things associated with divinity:
Knowledge of good and evil
Knowledge of the mechanics of life and the universe
Immortality (particularly immortality gained by feeding on lesser beings)
Sex and death
Many Yahoo! answers have already been given to the question of the apple on the cover of Twilight, but there are some possible versions of the answer that I haven't seen mentioned. There are two trees in the garden of Eden whose fruit is forbidden and could be symbolized by an apple, although the actual fruit in Genesis isn't specified. These trees include the tree of knowledge of good and evil and the tree of immortal life. After Adam and Eve become more like God by eating from the first tree, they are cast out of the garden because they might eat from the second. The process of becoming a vampire is associated not only with becoming more aware of questions of good and evil, but also with trying to become immortal. However, in Twilight there's a rebellion of two sorts because becoming a vampire means first taking the serpent's (devil's) advice to eat the fruit, then (in the case of Edward and other "vegetarian" vampires) trying not to sustain immortal life by feeding on people. Being a "vegetarian" vampire would seem like something forbidden to most vampires. Therefore, eating fruit can be considered forbidden both by the Biblical God and by the devil and traditional vampires.
The apple is also associated with scientific and carnal knowledge and discovery, particularly in areas which have clashed with religious faith. Bella and Edward meet while studying cell reproduction in Biology, one of the subjects most concerned with evolution and forbidden by churches. In the Twilight novel, Bella drops her books (repositories of knowledge) and Edward picks them up; in the movie, she drops an apple instead of books. Dropping something, or letting something fall, is a means of flirting (for instance, at some times in history, a woman might have dropped a handkerchief for a man to pick up). Bella, in that sense, can't help flirting, or acting as if she's flirting, because she's so clumsy. Neither Bella nor Edward can resist "falling" for each other. In the Bible, the fall of Adam and Eve is followed by Adam and Eve "knowing" each other, that is, having sex.
Bella is both mysterious and dangerous to Edward because she's his favorite intoxicant. She's food to him because he's a vampire, but may be considered poisonous to him, like Snow White's apple (the drug and Snow White references are both made by Stephenie Meyer, the first in the novel and the second in her discussion of the apple on her question and answer page). Bella is also the only human whose thoughts are forbidden knowledge to Edward in any significant way, since he reads everyone else's.
I have more to add in another post.
Apple- The apple plays along the line of Forbidden fruit thus Forbidden knowledge. Bella knows what she got into when she decided to start dating Edward. Instead of acknowledging the warning signs and running the other she stared in awe at the beautifully dangerous exterior (like the beautiful red shiny apple) and fell in love. The biting of the apple would indicate Bella's caving into forbidden knowledge.
Flower - The flower in the cover of this book is point down instead of up towards the sun and it seems to be covered in red which I took as blood. This would be an indication of Bella breaking up Edward and the long term effect. The petal that drops indicates how she bleeds constantly mentally.
Ribbon - The ribbon indicates the thin line between the feelings she haves for Jacob and Edward. Although Bella wants both of them to get along and she wants to be with both of them (of course in different ways) it's impossible and cause the threads between both emotions to completely unravel.
Chess pieces - This image could possibly mean the game of life Bella has to play. She had to protect her daughter first from Edward then from Volturi.
TWILIGHT:The apple on the cover of Twilight represents "forbidden fruit." because stephinie meyer 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............
NEW MOON:new moons cover page is RUFFLED TULIP which is indicating to the harsh experience oh bella which makes her flower dead...........