What to start reading by Szasz?
He said Insanity was his best book. I have read a lot of this one, although not cover to cover.
Suicide Prohibition: The Shame of Medicine might be the best to read if you will read only one. That's my opinion because I feel most strongly about that aspect of coercive psychiatry.
Psychiatry: The Science of Lies is good too. It goes into the history of psychiatry in significant ways. I think he does not treat PTSD fairly in it though.
The Myth of Mental Illness was his first book, I think. It's interesting although not direct and to the point like later books of his. If you search for it, you can read the essay version of this for free online. I recommend that essay.
The Szasz Quotationary is a collection of quotes by Szasz as the name implies. It gives a really broad overview of his ideas, although does not have as much details about each idea as many of his other books.
If you have to pick just one of these, I'd recommend Suicide Prohibition or The Szasz Quotationary. Suicide Prohibition will really show you how he thinks in depth about specific ideas related to suicide. The Szasz Quotationary will give you a broad overview about his writings. So which is best to start with? Depends what your preferences are I think. Both are available as electronic books and physical books.
Again, he wrote about 35 books.
If you prefer audiobooks, then probably start with Psychiatry: The Science of Lies since only about 3 of his books are available as audiobooks.