They didn't work jointly on the book....that was done by Clarke alone, however they did of course work jointly on the screenplay and Clarke used some of that collaboration for the novel. One thing that Kubrick did contribute was the title, that was his idea.....the following is taken from an interview given by Kubrick in the early 70's.....
"There are a number of differences between the book and the movie. The novel, for example, attempts to explain things much more explicitly than the film does, which is inevitable in a verbal medium. The novel came about after we did a 130-page prose treatment of the film at the very outset. This initial treatment was subsequently changed in the screenplay, and the screenplay in turn was altered during the making of the film. But Arthur took all the existing material, plus an impression of some of the rushes, and wrote the novel. As a result, there's a difference between the novel and the film ... I think that the divergences between the two works are interesting."
Also the book came out shortly after the film was released only to not take away from ticket sales.