For more Greek Mythology try The Cronus Chronicles by Anne Ursu! A super fun and adventurous series by local author and bff of Wild Rumpus Anne Ursu that doesn't flinch from the scarier side of Greek mythology: the underworld! If you made it through the twists and turns of Half-Blood Prince you'll love this thrill-filled series.
Rick Riordan Presents! If your kid is ready to venture into the world of global mythology, set them onto Rick Riordan Presents, an entire imprint of great books to explore, written authors of the cultures whose mythologies they cover! I personally love the series about Aru Shah by Roshani Chokshi, which stars a young story-teller and a lot of magic she didn't see coming!
For even more magic and adventure:The School for Good and Evil series by Soman Chainani. If you loved exploring Hogwarts in the first couple books, go back to magical school with the characters in this series! Past graduates of the the School of Good and Evil include all the fairytale characters you know and love (or loathe) but two members of the new generation don't feel they've been sorted right at all.
The Unicorn Rescue Society by Adam Gidwitz! If your nine-year-old is starting to read on their own and especially if they loved all the magical creatures of Harry Potter, get them into this super-fun series about a group of kids protecting all kinds of mythical creatures from around the world, be they dragons, unicorns, or even the Jersey Devil!