Minerva McGonagall was one of only a handful of people who knew, or suspected, how dreadful a moment it was for Albus Dumbledore when, in 1945, he made the decision to confront and defeat the Dark wizard Gellert Grindelwald.
-- J.K. Rowling (PmP)
The collection of three ebooks books collectively entitled Pottermore Presents. The books are:
Pottermore Presents: Hogwarts: An Incomplete and Unreliable Guide
Pottermore Presents: Short Stories from Hogwarts of Heroism, Hardship and Dangerous Hobbies
Pottermore Presents: Short Stories from Hogwarts of Power, Politics and Pesky Poltergeists
The books contain writings by Rowling also available on the Pottermore website, with a few odds and ends of new information. The most extensive additions are new writings about Animagi and Horace Slughorn.