Showdown (Sweet Valley High #19) by Francine Pascal

More than she bargained for...

When Lila Fowler's mysterious new boyfriend finally appears at her pool party, Jessica Wakefield is dazzled. Jack is handsome, sexy, and exciting—even more perfect than Lila described him. So Jessica wastes no time making sure he notices her, too.

Unknown to Lila, Jack dates both girls—Jessica during the week and Lila on weekends. But the sight of Lila's happy face on Monday mornings is more than Jessica can stand! Jessica's twin sister, Elizabeth, warns her to drop Jack before she really gets hurt. But Jessica would rather get Lila out of the picture. Then Jack will be hers—all hers.