With all the talk on the Frontier's threads, how can you not consider Ender's Game by Orson Scott Card?
Then there's Artemis Fowl for some light fun reading.
Warhammer 40k books are hit and miss.....make sure someone recommends you a book before you buy a random one. My bro loaned me Caphias Cain, Imperial Commissar which is quite an odd book from the rest of the WH40k series since its supposed to be kinda funny(?).
And another highly recommended series is the A Song of Ice and Fire series which i personally have not read, all my geek friends swear by it.
http://en.wikipedia.org/wiki/Game_of_thrones