We often define mental illness in terms of inappropriate beliefs, and risky as this may be on occasion, there is no doubt that some mentally ill people have bizarre beliefs that cannot be squared with any legitimate version of reality. One, largely implicit, justification for the study of the mentally ill is that such study might shed some light on those beliefs held by putatively normal people. Whether this is true remains to be seen. There are, of course, several books on mental illness generally and on thought disorders more specifically. Those listed below focus on the nature of Acrazy@ beliefs.
Rokeach, Milton The Three Christs of Ypsilanti. (*) Knopf, 1964. What happens when three mental patients in the same hospital each believes he is Christ. The book is less interesting than it might seem, but it still provides some interesting insights into the belief systems of the mentally ill.
Siegel, Ronald. Whispers: The Voices of Paranoia (*). Simon & Schuster, 1994. Basically a series of case studies of various paranoid people, it does not provide much by way of explanatory material. Quite readable and interesting.