The Sisterhood of the Traveling Pants, by Ann Brashares

Brashares, Ann (2001). The Sisterhood of the traveling pants. New York: Delacorte Press. 

Summary: Four best girlfriends spend their first summer apart from each other. On the eve of their separation, they ceremoniously designate a pair of "magical" jeans that fit all four of them perfectly as a way to connect them despite the distance.  They agree to take turns wearing the jeans and to mail the jeans along with a letter.  The story follows different plot lines to follow each girl's adventures:  Bridget falls for a soccer coach at her camp in Mexico, Lena tries to avoid a handsome young Greek man while staying with her grandparents in Greece, Tibby stays home in Washington, D.C. to work at a drugstore and surprises herself by befriending a young girl who is dying of leukemia, and Carmen's greatly anticipated summer alone with her bachelor father in Charleston is destroyed when she learns upon her arrival that her father is engaged and is living with his new fiancee and her two children.  

Additional comments:  Don't be turned off by the silly and melodramatic list of rules for the jeans at the beginning.  This "chick lit" gets better as it goes until you're flipping through the pages without pause and with tears streaming down your cheeks.  The well-developed characters show raw emotions and unpredictable behaviors as they are put into sad, challenging situations. Readers can continue on the characters' journeys by reading the three sequels in this series and catching the recent movie versions on DVD.