Well, I always liked the Danny Dunn books. And Val, I still love Encyclopedia Brown. smile

I've recommended Cyteen before, here, but was reminded that there is a lot of sex in it.

Possibly a little bit ahead of 7yo, but the Artemis Fowl books are terrific. And the Simon Bloom books, by Michael Reisman. Simon is 11, and I think Artemis is 12 when the series starts.