Stanislaw Lem. The Invincible is my personal favourite.
Read Lem if you like some food for thought in your books rather than just them being pleasant timewasters.
Just be careful to pick up one of his less-philosophical books, as these might be too much to handle for a casual reader. "The Invincible", "Return from the Stars", "Eden", "Fiasco" are good to start with. Those are S-F with a
very hard S part. "Solaris" fits in there too, but is somewhat more philosophical.
Should you ever read Carl Sagan's "Contact", and think it too soft on the Moch's Scale of SF Hardness, then try Lem's "His Master's Voice".
"Memoirs Found in a Bathtub" and "Futurological Congress" are in turn somewhat reminiscent of P.K.Dick's work, who incidentally is a great s-f author himself.
(Oh, and make sure to read English translations by Michael Kandel - he's a genius.)
Dick's novels and short stories tend to be very easily readable, while containing heaps of original and often disturbing ideas and worries. He was a gold mine in this respect, and the filmmakers are still digging that vein today - his ideas have been made into movies more often than any other author's(yes, it's not a hard fact).
There is that staple feeling of oppressiveness, of uncertainity of one's senses permeating his works that makes you question the reality, and the meaning of it all yourself, for days on end.
For starters, try "The Man in the High Castle", "Ubik" or "Flow My Tears, the Policeman Said". His short story collections are great as well.
As a fun fact - Lem thought Dick the only worthwile s-f author in the USA, while Dick thought Lem to be a false name of a KGB comitee hell-bent on gaining mind control of the public opinion, and even reported this "fact" to the FBI.
Dick was an interesting man.
Just as an additonal, quick namedrop, I'd also recommend:
Ursula K.Le Guin for her very strongly humane approach to her characters;
James Tiptree Jr.(aka Alice Sheldon) for her unflinchingly brutal dissections of what it means to be human;
Arkady & Boris Strugatsky for... I can't quite put my finger on it.
There's just something different, something unique about their writing style.