I agree with all of the previous suggestions. If your kid is into it, books like the Percy Jackson series are good stories but also easily supplemented with related Greek myths - and in my opinion the books are even better with an understanding of the mythology.
Also, I don't know about your DS, so YMMV, but with some of the books the darker or more emotionally intense aspects can either make the book a valuable reread (with a greater emotional understanding) or a book that should be held off for later (emotional OEs, anyone?). Also, sometimes an only moderately challenging or a comfortable book is valuable for the story alone (in our house it's definitely a balancing act, though).
If the wizard thing went over well, Septimus Heap is good and long (seven fairly lengthy books).