Szasz is not saying there is no such thing as human distress, he agress with that. It is just the labels and diagnoses that are attached to the person who exhibits a categorised "illness." Mental illness cannot be found in the body as a disease or a genetic change, therefore he sets out to highlight how psychiatry invented the disorers.
The basic belief being that psychological illnesses do not exist, as in a pathogene, or the body changing its infrastructure irrevocably. There is no bipolar, autism, schizo, drug addiction, alcohol addiction, anorexia gene. There is however a belief these genes exist and huge expenditure is placed upon finding them. All based on the perception that human beings are clockwork toys that lack certain chemicals just like a car or a robot. The views arrive from a sense of making the human being into a robot regardless of veracity. Szasz begins to talk apart the constructs within this book, piece by piece, and it makes very uncomfortable reading for any psych doctor, nurse etc.
The first and fundamental error of psych science, it all rests upon belief, just the same as religion. It needs adherents and people to spread the message. Then it needs to take out dissenters. Into the silo huge industries arise based on keeping the real causes of misery a mystery - the social structures created to make sense of the world and negating the emotional world at the expense of abstracts.
Szasz dons his armour, takes his sword and charges ahead into the mass ranks of the believers and they all melt into thin air, because their genetic beliefs are tissue paper. However this tissue paper is encoded in the belief systems of those who need to believe. It is this which keeps it all intact.
An insightful book written to take apart the modern fallacies of psychiatric "science" which is anything but.