Eclipse (The Twilight Saga, Book 3)
Plot summary

The story begins with the revelation that Seattle is being plagued by a string of unsolved murders, which Edward suspects are being caused by a newborn vampire that is unable to control its thirst. Edward and Bella fill out college applications, while Bella explains to Edward her desire to see Jacob, her werewolf friend, again. Meanwhile, Alice Cullen has a vision that Victoria, a vampire who is hunting Bella, is back in town. Although Edward fears for her safety, Bella insists that Jacob and the rest of the werewolf pack would never harm her, and he eventually allows her to visit Jacob once in a while.

A few days later, Bella expresses her desire to have Edward make love to her before turning her into a vampire. Edward initially refuses, explaining to Bella that he could very easily kill her. Eventually, upon realizing how much it means to Bella, he agrees to try in the future as long as they are married first. Despite having an aversion to marriage, Bella realizes that spending eternity with Edward is more important to her than anything else and accepts his proposal.

Bella and the Cullens realize that the murders in Seattle are being committed by Victoria and an "army" of newborn vampires. The Cullen family joins forces with the werewolf pack in order to combat this threat. As everyone else prepares for battle, Edward and Bella camp up in the mountains, where they are later joined by Jacob and Seth Clearwater, a young member of the werewolf pack, to wait out the fight.

In the morning, Jacob overhears Edward and Bella discussing their engagement and becomes very upset. He threatens to join the fight and get himself killed because Bella doesn't love him. To stop him, Bella kisses Jacob and realizes that she loves him, too. After Victoria and her army are successfully destroyed, Bella explains to Jacob that while she loves him, her love for Edward is greater and she cannot live without him. Jacob, angry at Bella's decision to become a vampire, runs away in his wolf form to escape the pain he feels.

[Source - http://en.wikipedia.org/wiki/Eclipse_(novel)]