The Sisterhood of the Traveling Pants series by Ann Brashares
This is another series that was one of the first series' I ever read. This was my first real foray into contemporary, and I sometimes forget that I actually read them. I've seen the movies so many times, and even though they aren't the most faithful adaptations, I still love them.
The series follows four best friends who find a pair of jeans that magically fit all of them, even though they are all different sizes and heights. This occurs right before the first summer that they will be spending apart, so they decide to keep in touch by sending the pants to each other throughout the summer, and documenting what amazing things happen to them while wearing the pants. Though the covers suggest light-hearted summer reads, these books do include some really serious, deep issues and topics like death, depression, broken families and first heartbreaks. But that's what makes them so wonderful; because they don't pull any punches. And though the girls spend the summers apart, they always find their way back to each other when they need each other the most.
Note: There is a "fifth" book in this series. It is an adult book set when the girls are adults. I've been spoiled for the main plot point, and I HATE that plot point so much that I have refused to read it. Though I've heard that they are adapting the adult book into a movie, so I'll probably see it, even if it kills me.
I recently realized that I don't actually own them anymore, because I'd finished the series back when I wasn't actually collecting books. So I'm looking into getting new copies again, because this is one series that I'd like to keep for a long time. Who knows? Maybe my daughter will get to read them one day.