There is a small preview article of JKR on the Today where she talks about the book, the character that got a reprieve and writing an encylcopedia. She had this to say about the encylcopedia:

The encyclopedia would include back stories of characters she has already written but had to cut for the sake of narrative arc (“I’ve said before that Dean Thomas had a much more interesting history than ever appeared in the books”), as well as details about the characters who survive “Deathly Hallows,” characters who continue to live on in Rowling’s mind in a clearly defined magical world.

It’s now time to sit back for a bit and enjoy the life that Harry has given her, Rowling said. And, when she’s ready, there’s always that encyclopedia waiting in the wings.

“I’m not going to do it tomorrow because I’d really like a break,” Rowling said, laughing. “So you may be waiting.”

There are spoilers, but, read the full interview HERE or click Read More!