In the first part of the novel the mood is scientific, anesthetized and sterile. The reader is taken into a world of test tube reproduction, cloning, hypnopaedia and the scientific creation of a society and its class system. First time readers of the novels are usually quite alienated from the introductory mood. They feel cold, alienated, and confused by the pseudo-scientific speech of the initial three chapters and for them to continue is a struggle.
There is no one single "mood" in the novel. There may be many. I have mentioned some, but there are others depending on the point in the novel and what's happening. When John the Savage enters the novel things change and become different. We no longer are casual observers and we begin to identify with this individual's plight to make sense of these two differing "worlds."