I was slightly younger than your niece when I picked up my first Heinlein book ("Space Cadet").
I am over 40 now, and have never stopped reading him. If pushed, I would have to say that RAH has been the author who has affected me the most.
Regards,
Yeah, Space Cadet would be another good one to get my niece. The author who most shaped my youth was Tolkien, but since I'm not allowed to give them Tolkien, I figured Heilein would suit just fine.
I don't see my brother & family but maybe once or twice a year. It would be really cool if I got to see this niece more often and talk with her as she reads these books. I think I'd enjoy it almost as much as she's going to!