Pottermore Presents currently three ebooks with collected extra writings from JK Rowling about the wizarding world. While they are not essential to the series, they provide just a little bit extra, which I liked a lot.
This first book collects the stories on Heroism, Hardship and Dangerous Hobbies. Some chapters were very strong, like the ones about McGonagall and Lupin. Others, like the one about Trelawney could have been improved.
Still, I enjoyed it a lot.