Well, The Sisterhood of the Traveling Pants, that is! :)
These seriously are my favorite series of books ever written. The bonds of friendship are so strong, even though they are tested over and over again. These girls became friends to me too as I read each summer of their lives, and then finally the last book, set ten years after the 4th book. And even then, when the girls had drifted as far apart as friends can be, they managed to find their way back to each other.
I'm a sucker for a coming-of-age story and these books fit the bill...even the last which takes place when they are in their late 20's. I truly believe people can grow and learn and change themselves even far into adulthood.