What is a metonymy or syncedoche in Twilight?
First, let's do some terminology defining. You probably know what a metaphor is (using a descriptor that compares two unlike things to mean something - for instance, "that little girl is like a bird."). A metonymy, however, is a concept, or a noun, or a piece of rhetoric not called by its own name, but rather something associated with it (Sir Francis Drake commented, " I wouldn't do this but the Crown wished it." He meant, of course, Elizabeth, but the Crown is symbolic of her power). Synecdoche is a bit more complicated, most grammarians refer to it as a subset of metonymy, in which there is a substitution of a part for a whole, e.g. "All hands on deck," of course meaning all people.
So, grammatically, we have:
1) Metaphor - changing a word from its actual/literal meaning to something descriptive but not exact - "like"
2) Metonymy - substitutes of cause for effect proper name for a quality, etc.
3) Synecdoche - substitutes a part for a whole
In Twilight, it seems as if our vampire hero, Edward Cullen, perhaps due to his age and sophistication, is the character who most often uses literative devises to make a statement, but more than that - give the tone and timbre of the conversation. It's almost as if the author desires to take us back in time to the classics of Romeo and Juliet, Wuthering Heights, Pride and Prejudice, and put us in a state of descriptive adjectives in which people spoke rather poetically to one another. For example:
You are exactly my brand of heroin. Comparing Bella to a drug that has historic references to addiction. Once someone starts heroin, they need more and more. Also alludes to the effect Bella has on Edward - like a drug, she makes him "high" - happy, content. Bella is his addiction.
Before you, Bella, my life was like a moonless night. This has many meanings. First, since most readers associate night and vampires with comfort, one could say a moonless night was ideal for a vampire. But now, it seems that everything was black, unseeable, and void until Bella came into Edward's life.
And so the lion fell in love with the lamb. Biblical and Aesopian reference that Bella, the innocent, pure lamb, is the object of love from the carnivorous, dangerous beast of the lion (Edward). Edward and Bella both know he could kill her at any moment and that in sociological terms they are of two different natures. Instead, the idea that a lion (war) could love a lamb (peace) is anachronistic.
Bella, your blood sings to me. Singing implies music, sound, it implies that the life of Bella (her blood) is music to Edward's ears and heart. And like music, it moves him into ecstasy.
Of course there are many more, and some critics have accused the author of using too many "stacked metaphors," but one must allow the author to have her own unique style and means of communicating to the intended audience.