Our family is deeply distressed by The Post's review of "On Being Mad or Merely Angry" by James W. Clarke {Book World, May 24}. It is just the latest in a seemingly endless stream of seriously flawed books and articles that viciously and unfairly exploit our son John. The best hope for predicting and preventing future attacks on prominent persons lies not in this book but in a better understanding of serious mental disorders.

Mr. Clarke and the reviewer, Lincoln Caplan, appear to be making a career of writing about John, but with little effort being made to present a balanced account or to give him the slightest benefit of the doubt. These self-appointed experts have never talked with John, and they obviously know very little about mental illness. It is inconceivable to me that anyone with an open mind could review the extensive trial evidence and testimony yet still say, "It is no longer useful to interpret these angry events as acts of madness, as the jury concluded." But, then, neither Mr. Clarke nor the prosecution had an open mind.

According to the National Institute of Mental Health, about 2.5 million Americans suffer from some form of schizophrenia. Based on his long history of health problems, it's very probable that John is one of them. The record clearly shows that he was so ill in 1981 that he could not control his thoughts or his actions when he committed his terrible deed. Thus, he truly was not legally responsible for what he did. One need look no further than to his pathetic motive to see that illness was the culprit; anger had little to do with it. We have heard from hundreds of other families who have told us they have a child with almost identical symptoms.

Nevertheless, John didn't get away with anything, as some think. For eight years he has been incarcerated in a maximum-security mental hospital, which in many ways is worse than a prison. Under the law, he is there for treatment, not punishment. He is trying hard to recover now, as many others have done. We ask that he be given a chance to do this, instead of being castigated repeatedly for becoming mentally ill through no fault of his own. JOHN HINCKLEY SR. Washington