They'd probably have been better of calling it the Wizard's stone, I guess. But whatever, it is what it is.

Loki, the first book could easily be read by an older elementary school kid (4th or 5th grade). When I read it (and was still a hater then, it took me years to give in) I felt it was very Roald Dahl-like; and I'd read many of his books when I was not yet 10. The older ones get much darker and more complicated. All of the books are in the 7-12 year range section at Barnes and Nobles.