Sci-Fi Books For Pre-Teens?
I did not watch any TV as a kid. Thus, my mother made me read books from age 6.
When I was ten or eleven, I had already read all of Jules Verne and some Kipling & Jack London books (those my mother judged suitable for a 10-year-old anyway). I loved those, and they turned me into a real book-lover.
I read a lot of mythology books too at that time (Iliad & Odyssey, that kind of thing).
By the time I was 14, I had read all of the then-available Stephen King (talk about dark stuff) and most of the Dune novels (politics? religion?) I stalled on the God Emperor of Dune and didn't read it entirely until I was 17.
All of that in English (not my native language, I started learning English at 13).
The Hobbit was my first English book, followed by the LOTR cycle and Dune.
After, almost all books I've read from american or English authors were in English. I don't know how old kids are when they start learning a foreign language in your country, but good reading skills are great for learning a new language.
Also some other books from Herbert, the Hellstrom's Hive comes to mind - I read that 20+ years ago, and still remember it.
'Destination: Void' too, and the sequels.
My mom kept me reading when I was a kid (simply no TV allowed during the weekdays), and I used to read 2 or 3 books a week.
That lasted until I was 18. Now, I read when I have the time. Still a lot, but nowhere near what I could read when I was 15.
Some that come to mind are :
'The Alien Years' and 'Hawksbill Station' by Robert Silverberg,
'The immortals' by James Gunn,
all the Dune novels and the additional work by Brian Herbert,
Michael Crichton's Andromeda Strain',
Paul Preuss's 'Venus Prime' series.
One that I love but would not recommend to a kid below 15 is Henlein's 'Stranger in a Strange Land'.
Definitely great (my favorite Henlein) but weird. I'm not sure I would have liked it as a kid.
Also a favorite : 'Hunters of the Red Moon' by Marion Zimmer Bradley (and the sequel).
As for fantasy, some good suggestions have been made above, and I would also suggest the Inheritance Cycle by Christopher Paolini (only two books out now, the third will be released in September). Addictive, and definitely kid material (although I read that when it was out, about 6 and 4 years ago).
Clancy, Len Deighton and a few others are good thriller writers too.