Breaking Dawm by Stephenie Meyer

To be irrevocably in love witha vampire is both fantasy and nightmare woven into a dangerously heightened reality for Bella Swan. Pulled in one direction by her intense passion for Edward Cullen, and in another by her profound connection to werewolf Jacob Black, a tumultuous year of temptation, loss, and strife have led her to the ultimate turning point. Her imminent choice to either join the dark but seductive world of immortals or to pursue a fully human life has become the thread from which the fates of two tribes hangs. Now that Bella has made her decision and her life seem ready to heal and knit together, could they be destroyed… forever?