As to her choosing to read below her level, she shows maturity on many levels (such as interpersonal relations, being polite, considering feelings, et cetera), but she is less mature than her classmates on things like boys, music, activities. She is an only child, we are 50, and there are no children in the neighborhood for her to play with after school. So, other than school, play dates with friends, and weekend activities (gymnastics, swimming, Mandarin language class, Sunday School) she is around adults more than kids during her free time and her development has probably not been as complete as if she would be outside playing in the yard with kids her age.
Goody
Don't beat yourself up so much about opportunities with other kids. A lot of the things you reference, boys, music and activities are driven by shared pop culture. Our kids often aren't interested in that - so even if she was surrounded by other kids she might still be interested in whatever she is interested in. Sometimes it takes awhile to find the right reading niche - I find I am often switching around between fiction and nonfiction, finding the right level of where he is now, or where he is going. And I spend a lot of time reading the book threads on the site!
Plus its typical for the gifted to enjoy adults more than age mates. My DS 5 makes up these elaborate games to make their ordinary games more interesting to him - like tag - they are all playing tag - he is firing shark torpedoes and bringing evildoers back to his lair. Some other little boys make up his "team" and I am not even sure they know he is doing this or are actually playing with him!
