I havent read the book so i can only speak for the film.
The monolith is a signifier. It is used in the film as not only an tangible object but also a means of presenting an idea and concept.
In its tangible form it is used as a transmitter and conduit from which a message , that is apparently undecipherable, seems to have almost apocalyptic consequences.
This message seems at first to instigate the passage of Man from a benign beast into an intellegent being.
When it appears again it seems to signal the transistion of Man from a sentience to a higher, more ethereal, state of consciouness.
In its less tangible form the monolith seems to represent the Creation. This part of the story is less difficult to interpret and is open to argument but the image of the foetus, after the apparent death of the main character, seems to lend weight to the overall idea that this is the Resurrection and that we have entered into a new epoch within the Creation.
"My God - Its full of stars!"