I think the Northern Lights books are wasted on an 8yo. DD was an excellent reader and enjoyed reading them at that age, but she has re-read them recently (she is nearly 12) and got so much more out of them. There are some easier Philip Pullman books though.
DD really enjoyed Skellig (and My Name is Mina) when she was around 10. I think it might be a bit sophisticated for an 8yo though. I think the Eragon series might also be a bit too complex for an 8yo.
The divergent trilogy/hunger games are great books but quite dark for such a young child. Maybe wait a year or two?
The Lemony Snicket books were great for dd at that age. Also the Pseudonymous Bosch books, the Alex Rider series, Percy Jackson etc.
My dd was also a big fan of the old classics when she was 8ish. She loved Little Women, the Anne of Green Gables series, Little House on the Prairie, various Noel Stretfield books, The Secret Garden, A Little Princess etc. Heidi was a particular favourite. The language in some of these books is quite challenging but the themes are still age-appropriate.
As for Harry Potter, no need to feel bereft! If she is anything like my dd, she'll probably re-read the whole series several times over, so there will be plenty more enjoyment from them yet. 