Slow Dance by Rainbow Rowell

Back in high school, everybody thought Shiloh and Cary would end up together - everybody but Shiloh and Cary. They were best friends, that's all. But, somehow, everything changed. Both left town - took different paths - and suddenly it's fourteen years since Shiloh last spoke to Cary. She's been married, divorced, and is back living in the house she grew up in. But when she gets invited to an old friend's wedding, all Shiloh can think about is whether Cary will be there - and whether she's dreading or dreaming that he will be.