My sons enjoyed Rudyard Kipling at that age. [Rikki Tikki Tavi] and others.
We read the Chronicles of Narnia and really enjoyed them.
I loved the boxcar books when I was that age.
But I have a passion for nonfiction, and when I was in the third grade started reading juvenile biographies. My sons also loved to read science type books, nonfiction books about animals. Almost anything would hold their attention. One loved reptiles and the other loved sea creatures [whales, sharks, dolphins and not in that order].
I'm not a big fan of fiction works in general, especially modern subject fiction. They just are not that well written. Didn't hold my sons' attention.
Might try Dickens' Christmas Carol later in the year. He is my favorite author bar none, and that particular story is fascinating even for younger readers.
A book that is challenging for your mentee, that you might have to help her with some of the words, might push her along to bigger and better things. She might love reading more if there were big ideas in the books, something important, something to make her think. Not just reading to be reading.
You and I are on the same shelf with nonfiction and biographies, and reading books with big ideas. The latter is my goal. Also, I take the time, when we reach a word I'm pretty sure she doesn't know, to inquire, and if it's new, I explain it.