Why you must read them.
1. At the Mountains of Madness, by H. P. Lovecraft (1936).
Why you must read it. Lovecraft is known for his horror stories, however, this is a great science fiction novel that exists in his cosmicism universe. Also, if you get the winks, you'll discover it is a response to Poe's 1838 novel The Narrative of Arthur Gordon Pym of Nantucket.
2. Childhood's End, by Arthur C. Clarke (1953)
Mankind is entangled with shallow conflicts among nations when, out of nowhere, huge alien ships appear over the skies of major cities all over the world. They claim they come in peace, to make sure humans don't destroy themselves, and to do that, they will guide Earth civilization from now on... to their end.
Why you must read it. This is the original novel that has inspired all other stories, movies and television series about huge disc shaped ships invading the cities of Earth.
3. I am Legend, by Richard Matheson (1954)
Why you must read it. This is not just a regular "zombie apocalypse" story, as many have been lead to believe because of the horrible film adaptation (Which in fact it is the adaptation of another adaptation of the book, The Omega Man), this is the point of view of a regular guy who sees his world changed upside down and must adapt in order to survive. The title of the book is explained by an astonishing end, one of the best out there.
4. The Martian Chronicles, by Ray Bradbury (1950)
Men always dreamed and imagined how would their first contact with aliens would be, but when it finally happened, in the red planet, it was nothing like they thought, and neither was the second, the third or the subsequent. The book is composed of several stories that depict the interactions between humans and Martians during the colonization of Mars, many are almost surreal, with a spirit of nostalgia in every page.
Why you must read it. Humans often think they know everything, Bradbury proves here that humans don't even know themselves. One of the most beautifully written science fiction books, it doesn't only transmit ideas, but clear feelings and atmospheres that transport the reader directly to the neighbor planet.
Humans are in space now, they are capable of interstellar travel and have many colonies, however, their lack of faster than light technology makes their ships victims of the relativistic effect known as time dilation. This effect makes it possible for the soldier William Mandella to witness the entirety of a war between humans and an alien race that spans for centuries.
Why you must read it. The novel reveals the true nature of man and it's consequences when he goes into space. This is not just another science fiction war story, it's a great drama and a philosophical critic towards war and it's long term impact in society.