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)
PmP is a collection of three e-books, collectively entitled Pottermore Presents.
The e-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, with a few extra paragraphs about Minerva McGonagall.
The titles of the e-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
From the Web
Tumbler (JonSnowWhite): Animagi by J K Rowling
Harry Potter Wiki: Pottermore Presents
WizardingWorld (Pottermore) news feature: How Pottermore concocted Pottermore Presents