My brother Ger was really into the Oz books from ages 5 thru about 8. And, I must say, from what I remember they're quite a lot of fun.
I like the Series of Unfortunate Events books, but mostly because they're a long parody of "penny-dreadful" melodramas and I think they're hilarious. I'm not sure that just any 8-year-old would like them; they're short enough that you could pick one up to figure out whether your kids will appreciate them or not in the space of an afternoon.
You can also try The Phantom Tollbooth, which I read at around 8 or 9 years old (it was a suggestion on our summer reading list) and have on my "to re-read" list now.
Our now-10-year-old niece was crazy for Roald Dahl and the Beverly Cleary Ramona books, but I'd also suggest Judy Blume's Tales of a Fourth-Grade Nothing series about Peter Hatcher & his trials and tribulations with his little brother, Farley Drexel "Fudge" Hatcher. (I think I was about 8 when I read that.)
My dad & I read 3 of the Chronicles of Narnia together when I was 6 or 7 - we started with The Lion, the Witch and the Wardrobe and moved on to Prince Caspian & Voyage of the 'Dawn Treader', though I think newer sets have renumbered the stories so that they're read in "chronological" order and begin with The Magician's Nephew instead. (Not my preference, by the by, but it's not like it doesn't make sense.)
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Liz
DD (3/2010)
"Make mistakes! Get messy!" - Miss Frizzle