Twilight is the first book in the Twilight Saga.
Twilight was first published in 2005 and has come to be more successful that the Harry Potter series.
Summary, as seen on the copyright page of Twilight, first edition:
When seventeen year-old Bella leaves Phoenix to live with her father in Forks, WA, she meets and exquisitely handsome boy at school for whom she feels an overwhelming attraction, and who she comes to realize is not wholly human.