Look for Lloyd Alexander's
Prydain series. The first book is
The Book of Three; it should be entirely suitable for an 8-year-old. As the protagonist grows up, the books become more complex; they are aimed at readers whose age is similar to (or at least
close to) that of the protagonist. Thus, the fifth book in the series is probably really aimed at teens rather than younger children. The second book in the series was a Newberry Honor book; the fifth and final book won the Newberry Medal.
Really good stuff. Highly recommended for adult readers as well (with the caveat that the early books clearly show their focus on younger readers). Not available in legal ebook editions that I can find.
Xenophon