Given *my* SF preferences I'd like to start by suggesting the Robert A. Heinlein Juveniles over at Baen.com but they haven't yet gotten to releasing the most appropriate one for a precocious 8 year-old: "Have Spacesuit, will travel". Maybe soon it'll be out in a legal ebook release...
In the meantime, take a look 'round here:
http://www.munseys.com/detail/mode/cat/20/Children
You'll find the OZ, Alcott, Lang books and a lot more, ready to go.