People occasionally ask me what Thomas Szasz's best works are. The optimal introduction is The Untamed Tongue. It's a book of aphorisms that cuts to the heart of his philosophy of mind. If it doesn't make you worry that there's something fundamentally rotten in the psychiatric worldview, nothing will.
If The Untamed Tongue piques your interest, but you want arguments instead of aphorisms, then I suggest you move on to Insanity: The Idea and Its Consequences.
P.S. Check out the press release for the 2005 Szasz Prize. The prize ceremony is 6 PM Wednesday, September 21st at the Harper Library at the GMU Law School in Arlington. If you live in the D.C. area, you'd be crazy not to be there!