"I know this is a weird thought. But, I guess I'm visualizing this girl in her 20s or 30s thinking back and saying, "You know, when I was 9 years old, some stranger gave me a book that changed my life. It made me who I am today.""
Well, if that's what you're going for, try "A Clockwork Orange".
Kidding, folks, kidding!
I'll second the Little House series. And of course, some of the classics aren't bad. "The Phantom Tollbooth" by Norton Juster was a big one for me. Shel Silverstein is also a good choice (although I'd avoid "Uncle Shelby's ABZ Book" if I were you).