I started reading this series with my son when he turned 4. We'd just started reading chapter books and I was looking for great stories without the presence of annoying pop culture. The Oz books were recommended by a trusted friend and I was skeptical at first, especially at his age, but the first book is so much better than the movie, and each subsequent book has been better than the last. After checking a couple different versions out from the library we now make sure to always get the books with the original illustrations by John R. Neill (the Dover 1980s and 90s reprints are the best). I don't know what we'll do when we reach the end of the series!