If your interested in some older school sci-fi I can't recommend Asimov's 2 collection of short stories enough. They were rereleased with Asimov's commentary on each story which are almost as interesting as the stories themselves. Starship Troopers is my favourite Heinlin, simply because its a diatribe on creating the perfect fighting force and he makes some great points. It was really too bad they destroyed it with that stupid movie.

And while note really older I love the sci-fi of Cory Doctorow and his books are mostly set in Kensington, its nice to read a good Toronto writer explore the city. Also Orson Scott Cards Ender Trilogy and his Ender's Shadow series as well. Some great philosphy in those books.