James Morrow 
Continent of Lies 

Cephapples can seriously damage your health…A cephapple is a dreambean — a programmed hallucination. Quinjinn is a reviewer of cephapples. When he is persuaded to sample one particular dream, it is so real that he cannot get it out of his mind, and so hideous that he cannot remember its climax. Horrified when his young daughter is exposed to it as well, Quinjinn resolves to find the tree on which this cephapple grew, and to destroy it before it can poison any other minds.His interstellar quest becomes a bizarre journey through a cold and alien galaxy and exotic distant planets — and into the deepest recesses of his own mind.