The Sisterhood of the Travelling Pants + The Second Summer of the Sisterhood by Ann Brashares

Carmen's the one who found the jeans, in a second-hand store. They didn't look all that great: they were worn, dirty, and speckled with bleach. But Tibby liked the look of them. And then Lena tried them and Bridget. And that's when the magic began ... because the pants fitted each of the four girls perfectly. Not 'sort of okay', not 'just about right', but PERFECTLY. Even Carmen (who never thought she looked good in anything) admitted the jeans looked great on her too. And over a few bags of snack food and some bad eighties music, the Sisterhood of the Travelling Pants was born ...