Actually, it's called "Are You There God? It's Me, Margaret." I had a hard time remember, too. So, I looked it up! It's mainly about a girl's rite of passage into adulthood and her relationship with God. Here goes...
"Margaret Simon faces a lot of changes the year she enters sixth grade. First, her family moves from New York City to a small town in New Jersey where she must settle into a new school and meet new friends. She also deals with her questions and anxieties about her first period, her first bra, and her newfound feelings for boys. A virtual rite of passage for preadolescent girls, ARE YOU THERE GOD? IT'S ME, MARGARET was originally published in 1970, and was one of the first works of fiction to address the topic of menstruation. This book also broke ground by examining religion in a decidedly secular manner. The daughter of a Jewish father and a Christian mother (both of whom come from deeply religious backgrounds, but neither of whom is particularly religious), Margaret's personal struggle to determine where her own faith lies mirrors her adolescent struggle to establish her own identity while still fitting in with her peers. "
Perhaps it's the issue of examinig faith that makes it relevant here?