(Nos #3 (The Redbreast) and #5 (The Devil's Star) have already been published and #4 (Nemesis) is due next month.)
At the official Jo Nesbo website (written in English) there's quite a lot already on the site even though it's still under construction. It confirms the book order and titles and there are links to extracts from the books, though these aren't available yet. There is a creepy trailer for The Snowman.
There's a long interview too which refers to book eight and concludes with:
How many Harry Hole books are there in you?Read the interview in an easier pdf format here.
“I’m not absolutely sure, but he’s a hero with the seeds of destruction in him. He won’t live forever. He is going to escape from Oslo in the next book. To Hongkong.”