When a large magnetic anomaly is discovered on the moon, humanity uncovers an ancient observer of life in the solar system that has been guiding human development for millennia.
Was written alongside the film: 2001: A Space Odyssey
The beings which created the monoliths have long-since abandoned physical forms and instead live as beings of energy on the surface of their star.
The monoliths were created by a highly advanced civilizations, and until the end of the novel, have unknown purposes and motivations. When astronaut David Bowman enters the "Stargate" he sees brief glimpses of the massive artifacts, like spaceports, left over from previous stages of this civilization.
The civilization that created the monoliths created many and sent them to planets with potential for intelligent life. The monoliths induced intelligence in humanity, for instance. By the time of the main events of the novel, they seem to not have much presence.
HAL 9000, the computer tasked with running the ship Discovery, is tasked with simultaneously always providing helpful information, while also keeping the true nature of the mission secret. To resolve this contradiction, he decides to kill the crew members.