Books for Girls! Damn, pretty much every single thing I was going to recommend is on here! I was like "surely no one's gonna say Boxcar Children" but then BOOM, bocko, there you go, I shoulda known.
I was definitely reading Stephen King in 3rd grade, and with no namby-pamby censoring of pages either!
I think (with that in mind, I mean, considering I am obviously not some sort of genius) that the 10 year old mind can really rise to the occasion. For that reason I would not shy away from something like The Hobbit even though it seems long and complex. I have very intense memories of having it read aloud to me as a 5 year old and totally digging it--surely 12 can go deep. Definitely I had read it again by myself by 12. It can skew young because there's SO LITTLE PSYCHOLOGY. Just heroic deeds, bad guys and good guys, and cool beasts. Also MAPS.
Evie, very similarly, at age 9 I read a nuclear holocaust book that obsessed me more than probably any other book I've ever read. I still think about it probably weekly, and can see the cover vividly. There's a part where they have to shoot the family dog. Then everybody dies of radiation poisoning. YOUNG ADULT SECTION OF THE LIBRARY.
There was a Drew-like series for young girls called Trixie Belden that I was furiously into. It's like an updated Nancy Drew--way fewer dresses and fat jokes. The main character is a tomboy. I remember in particular one scene where she wears a dress to impress a boy and everyone is like "OMG LOOK AT YOU IN A DRESS" and she's so filled with rage, it really struck home with me. "My god it's like she's known me all my life"
Also!!!
ENCYCLOPEDIA BROWN!!!!!!!!! Riddle-solving pre-teen sleuth!
And The Great Brain, which was about turn of the center Jesuit school boys and a weird genius who played clever pranks. You can read the book then play the pranks on your own friends (they are harmless and largely math based). There's a scene where they see a basketball for the first time.
Ralph S. Mouse, isn't that by Beverly Clearly? The mouse drives the car by saying the word "Vrooom!" WTF?!
At her age I also read The Plains of Passage series but I don't recommend that necessarily (best kept secret of my generation's girlhood: totally pornography. But your parents would still get them for you for christmas because they seemed vaguely education and certainly did not indicate their porniness up front. Such good memories though)