(Quotes from the article in Wikipedia on the book)
The monolith is obviously the product of a much higher intelligence than humans.
In the first chapter of the movie, the apes that touch the monolith become more intelligent. In a single stroke they learn the use of tools. If you see the confrontation with the other group of apes you can notice that they stand and walk more straight as well. And you see also the strongest impetus for technological development: the possibility to create weapons.
The book shows one such monolith appearing in ancient Africa, 3 million years B.C. (in the movie, this was altered to 4 million years), where it inspires a starving group of the hominid ancestors of human beings to develop tools.
In the second chapter, when astronauts visit the monolith on the moon (not clear if it is the same one: it was found under ground, due to its anomalous magnetic field), the monolith produces a strong radio signal, which, we later are told was specially aimed at Jupiter. So again: it is some piece of technology, registering that is was revealed, and that humanity is ready for the next acceleration in their development.
They arrive just as sunlight hits upon it for the first time in three million years. It then sends a piercing radio transmission to the far reaches of the solar system. The signal is tracked to one of the moons of Saturn
(In the movie it is Jupiter).
In the final chapter the space ship nears a monolith near Jupiter which seems much bigger than the other one(s). Bowman seems to disappear in it, entering other dimensions of space and time, growing old and reborn as a ‘starchild’, approaching the earth.
So my interpretation is that the monolith is a technological construction from a very, very much higher intelligence (God-like?) that took influence on the development of humans at least twice: in the beginning scene helping humans to become much more intelligent, in 2001 to produce the starchild for the next acceleration in development, whatever that maybe.
The book goes further:
He decides to go out in one of the extra-vehicular pods to make a closer inspection of the monolith. Programmed for just such an occurrence, the monolith reveals its true purpose as a star gate when it opens and pulls in Bowman’s pod. Before he vanishes, Mission control hears him proclaim: “The thing’s hollow—it goes on forever—and—oh my God—it’s full of stars!”
Bowman is transported via the monolith to an unknown star system. During this journey, he goes through a large interstellar switching station, and sees other species’ spaceships going on other routes; he dubs it the ‘Grand Central Station’ of the universe. Bowman is given a wide variety of sights; from the wreckage of ancient civilizations to what appear to be life-forms, living on the surfaces of a binary star system.
He is brought to what appears to be a nice hotel suite, carefully constructed from monitored television transmissions, and designed to make him feel at ease. Bowman goes to sleep. As he sleeps, his mind and memories are drained from his body, and he is made into a new immortal entity, a Star Child, that can live and travel in space. The Star Child then returns to our Solar System and to Earth. Once there, he detonates an orbiting nuclear warhead. Like Moon-Watcher three million years before, the Star Child is now master of the world and uncertain what to do next—but like Moon-Watcher, the Star Child too will think of something.