Meyer argues that positive thinking is a dead end, as it fails to grapple with social problems or the need for political change. His path to that argument is challenging. He jumps around in time and writes in an impressionistic, essayistic style, so that the book feels like a bebop or jazz riff on the history of positive thinking instead of a straightforward, footnoted narrative. I'm not sure how good of a history the book is.
Meyer situates the need for positive thinking in weakness — the desire to rise above one's problems — but I think he underestimates the importance of metaphysics and pursuing supernatural power in driving the mind cure movement. (See Catherine Albanese's A Republic of Mind and Spirit for a definitive treatment of metaphysics.) Metaphysics, in my understanding, could involve power as well as weakness or illness. Other problems: Meyer underestimates the size and imagination of the nineteenth-century women's rights movement; his description of nineteenth-century feminists as a "handful of articulate women" is sexist and reductionist. Time has disproven Meyer's prediction (from the 1980 edition) that positive thinking might lose some power in the 1980s. Donald Trump proved the endurance of positive thinking.
Tl;dr: Contains Pithy observations about the psychology and drawbacks of positive thinking, but has dated language and structure.