good places to start are
Enders Game by Orson Scott Card
Heinleins Juveniles
http://en.wikipedia.org/wiki/Heinlein_juveniles
Old Mans War by John Scalzi
Arthur C clarks Childhoods end and Rendezvous with Rama
all those books should wet your taste for more. all are considered classics, besides Scalzi which is Contemorary, but he is a great new author, who is a pleasure to read. I think he's got 6 or 7 books out, 4 of them from the old mans war series (all but the last one are self contained and don't require reading any of the other ones. the 4th one takes place from another characters POV from book 3)