Hey, sorry I've posted in this thread thirty times already, but I happened to talk to my mom on the phone tonight -- she's been a teacher for 30+ years, and her favorite thing (and her specialty) is teaching reading!
So you're darn right I asked for some recommendations. Just off the dome she gave the following, and promised to poke around in her collection of kids' books for more:
Nat'l Geographic Kids Magazine - (that's a direct link to the subscription page,
here's their main wobsite)
The
Geronimo Stilton series - she mentioned that this series in particular plays with language a lot, lots of idioms and such, which might make it more challenging or more fun to read.
Junie B. Jones books
She also mentioned - and I'm sure your niece's reading teachers have told her folks about this, too - but if she's still having trouble getting through sentences or pages of books, being read to aloud while she follows along on the page, or "partner-reading" where they take turns reading sentences or paragraphs can be helpful.
ETA: I suddenly remember lots of girls in elementary school reading the
Babysitter's Club books. Used to buy them by the bushel basket from the Scholastic Catalog and at the Book Fair.