This is a question I get asked all the time working in the bookstore: "What would you recommend for somebody who loved "_________"? (fill in the blank)" Usually I can come up with something, but that something can be a bit of a wild guess if it's not a book or genre I normally read. This is where you helpful people come in. If you have a suggestion, I'd love to hear it.
It's also safe to guess that books by the other authors in this series are good guesses - especially Rick Riordan's Percy Jackson series. Beyond that, however, I'm not entirely certain what else to suggest, although I'm thinking that the various spy/thrillers designed for kids might catch their interests: The Young Bond series or the Alex Rider books for example.
What would you suggest?