A bit about the new book:
Yes, it is considered the eighth Harry Potter book.
Rowling was involved with the creation of the story itself, but is not the writer of the newly released book. Instead, this book is a written play created by Jack Thorne, a British writer and producer of award winning shows and movies. This book is written as a screenplay–if you've never read a screenplay, just FYI–it takes some getting used to. There isn't any internal dialogue, and readers may regret the lack of detail. But, hey, it's Harry Potter.
Harry Potter and the Cursed Child is now playing on a London stage.
What's it about? Well, Harry is all grown up now, and the story actually picks up in the train station scene that you might remember from the end of book #7, in which Harry and family are dropping off son Albus for his first year at Hogwarts. Without providing spoilers, I will say that the story is about coming to terms with living up to the reputation of being the Boy Who Lived, and dealing with being the son of the Boy Who Lived–both challenging roles.
PS–PUT YOUR COPY ON REQUEST NOW!! Seriously.