'The Black Cloud' by Fred Hoyle, in 1957 highly praised by Carl Sagan, the famous astronomer and astrophysicist, who called it one of the best SF books he had read. Carl Sagan himself of course wrote the well-received 'Contact' in 1973. The brilliant and prolific Arthur C. Clarke wrote many wonderful books but I found 'Rendevous with Rama' tremendously exciting. it had a sense of reality, that made me imagine that it really was 'first contact' but I have to say that although I enjoyed the sequels I found them slightly disappointing. The imitable Isaac Asimov is known for many great works, but again from a purely personal view the collection of robot stories in 'I, Robot' 1950, and the introduction to the Three Laws of Robotics' is one of those most fondly remembered. I have a copy before me and the wisecracking remarks between Donovan and Powell - the 'Holy Space! and 'Jumping Jupiter!' are of a different era and recall the early sci-fi magazines in the US. I would be interested to hear other opinions.