Poul Anderson was a Grand Master; he produced an awful lot of SF and fantasy,
and most of it is worth reading. Tau Zero is about an interstellar space
ship on which essentially the brakes fail, and they keep on accelerating and
accelerating..... Tau Zero is the figure that represents exactly how close it is
possible to come to the speed of light. I also recommend his THE HIGH CRUSADE
and THREE HEARTS AND THREE LIONS. I used to know him when I was a grad
student at UC Berkeley (back when the earth's crust had barely cooled); he was a
very nice man indeed. He used to give a party every year on Bilbo's Birthday at
his place in Orinda. I generally prefer his fantasy; others will probably
have stronger recommendations in his SF.
tal