You can see the covers in the article on Pottermore. They were created by Miraphora Mina and Eduardo Lima (MinaLima), the graphic design team behind the artwork of the Harry Potter films. They're indeed quite pretty.
The descriptions of the eBooks read:
Hogwarts: An Incomplete and Unreliable Guide dives into the nooks and crannies of the beloved wizarding school itself, unfurling everything from details about the Hogwarts ghosts, to what happens when the Sorting Hat can’t decide on a student’s house.
Short Stories from Hogwarts of Power, Political and Pesky Poltergeists explores the darker roots of the wizarding world. You’ll learn more about the troubles that faced the Ministry of Magic, the horrors of Azkaban prison and enjoy an entirely new original piece of writing by J.K. Rowling on Horace Slughorn.
Finally, in Short Stories from Hogwarts of Heroism, Hardship and Dangerous Hobbies, Pottermore celebrates the colourful staff of Hogwarts. There’s more to discover about Care of Magical Creatures teacher Silvanus Kettleburn, as well as another new piece of writing from J.K. Rowling on Minerva McGonagall, and her involvement in the second wizarding war.
The Pottermore page does not list a price, but if this other article I found is to be believed, they'll be $2.99 apiece. Nowhere as expensive as any other HP book so far (Quidditch Through the Ages is probably closest to that price) so it has me wondering how short these will actually be.
Anyone excited for more material? I think I may give them a go, as the Cursed Child got me craving for more HP reading.