I'll second Enders Game, I read it when I was a teen and I just picked it up a bit ago for my son to read.
I'd also recommend The Sherlock Holmes books. You can pick them up for free over at project gutenburg. They're somewhat predictable but I think that's because every mystery novel is somewhat influenced on them. And even then, they were quite an enjoyable read. My daughter and I raced to see who could finish them first and see how many we could guess the mystery in.
The I Robot series is also a great series. There are a lot of really interesting ideas in the books that don't specifically have to do with robots. A lot of gems that I think get passed over because they're not the theme of the book.