I remember the first time I read Lock and Key, when I was a teen this book made me fall in love with reading, even now in my late 20s this is still one of my favorite Sarah Dessen books. It gets me in my feelings, makes me think of my own troubled relationships, which is probably why I love this book so much. 
Reading it now I do think the format of the book is a tad weird, she uses flashbacks to set up small scenes, sort of like Ruby being lost in thought and thinking back to what happened a few days ago, to catch us up instead of drawing out scenes that aren't majorly important.