If you are disturbed by the idea that to grow up is to learn to live with disillusionment, if you are fascinated by the perplexity of child-rearing, or if you fear you were more creative as a child, The Beast in the Nursery offers an illuminating and possibly life-changing experience.      In four interrelated essays, Adam Phillips arrives at startling new insights into issues that preoccupied Freud, showing in the process that far from having lost its relevance, psychoanalysis is still one of our most incisive tools for the exploration of the human psyche and its possibilities.  Phillips transforms the genre of the essay into an instrument for intellectual investigation of the most absorbing kind.