Tension and Suspense
Tension and suspense are the general moods of Eclipse. The main character is always waiting for something bad to happen, but you don't really know what's going to happen. There is always that feeling of tension between the vampires and the werewolves, and because Bella is in the middile of them both, and because she is the main character that means that there is even more tension.
The author creates this mood by having an element of the unknown written into the book because you know that there is something bad that is going to happen but your not sure about what it will be exactly. Also, she creates it because at the end when there is the fight between the Quileutes and the Cullens against the army of newborn vampires, Bella is not in the fight and the way she thinks and what she says builds up the suspense of finding out what is happening in the fight.