Like much classic sci-fi, the characters move in service of the idea but when that idea is 'what if aliens nudged humanity down the path of evolution and are waiting to do it again', I barely mind. 

The infamous HAL section is almost a bonus short story in terms of the novel's structure (though thematically a microcosm of the larger idea) yet it packs in such dread, tension, and humanity that it almost feels like a flex. 'Saggy middle', who?

Anyone left wanting by the film's ambiguity will also be grateful for having read the book. It provides a lot more explanation while still maintaining the sense of scale and wonder thanks to the nature of the idea - a firmer platform to leap off of.