My understanding is that 2001 isn't so much an adaptation of the novel as it is a script that was written through extensive collaboration between both Kubrick and Clarke, eventually adapted into both screenplay and novel. I've got quite an interesting book that goes into some detail about it - "2001: filming the future"

I think it may have been in there that I read something to the effect of "Kubrick withheld his approval of the book because he didn't want to hurt the release of the film."