J.K. Rowling, along with many other authors including Nick Hornby and Neil Gaiman, contributed an 800 word story to this book for charity. It centers on James Potter and Sirius Black about three years before Harry was born according to Jo. It's hand-written. It's brilliant. And she said it's from the prequel that she's NOT working on. But my fingers are crossed that she will want to now that she's delved into that world a little bit.
I just pre-ordered the book, but you can read all of the stories online here. Proceeds go to charity if you buy it.