You want to find out what sort of thing interests her.
I speak as a mum of 3 voracious readers, (14yo, 11yo, 7yo) but they pretty much read totally different things. The only books they've all loved are the Arthur Ransome books, but even then they've all preferred different ones.
Dd1 likes hard mysteries, spy stories, adventures, fairly modern books but needing to be enough close enough to realistic not to have to suspend too much belief. (Coot Club/Big Six by Aurthur Ransome)
Dd2 likes gentle stories, more "real life" or magical stories. She won't touch anything she deems to be scary. She very much likes 50s/60s Girls Own Type, where the worst you're going to have is a mountain rescue. (Swallows and Amazons, Swallowdale, by AR)
Ds likes factual most, but WWII stories, or other historical stories, but also some mystery/adventure, but old fashioned ones. He loved Peter Duck by AR.
If I find a book with a marker in it then I can tell immediately by the content whose it is likely to be. A book one has been bored with, another may well love. Enid Blyton is probably the only exception-they've all loved Famous Five and the Adventure Series.
They also read in a different way. Dd1 may have 3 or 4 books on the go. But when she's read them she rarely goes back (except Watership Down which is her comfort read) and will exhaust the series and then move onto a different series.
Dd2 reads books again. She can finish and just restart at the beginning-up to 4 times is her record. If you take one book away she'll just pick up the next and continue.
Ds likes reading with me. He likes me to read some, and he continues. He also play acts them out and goes back and rereads his favourite bits over and over again.
I would go for one that you think is slightly below their reading age. Simply because they have to really love it to struggle through one that's a bit hard.
I'd also suggest you go to the library (or a second hand shop may have some) and get a couple of Horrible History books and put them by her bed. Bedtime at 8:00, you can read until 8:30 goes down well in our house. Of course if she's engrosed you may choose to let her read beyond that. 
And then choose a book (like the Awful Auntie) that you can read together. Again leave it by the bed, so she can read on. I don't actually know that book, so I can't comment, I think dd2 has read some of his, but dd1 thought they were totally boring.
Ones mine have loved at age 8yo:
Roman Mysteries, Alex Rider, Chalet School, Dolphin Summer, Ballet Shoes, Cue for Treason, When Hitler Stole Pink Rabbit, Little House books, My story series, Lone Pine Series, War Horse, Butterfly Lion.