Definitely the most in-depth look at the art of psychiatric patients I've read in the decade or so I've been interested in the topic. To be clear, this book is really about how and why "insane art" became an object of interest at different points in history from Romanticism until the 1970s and it focuses very heavily on Europe and especially England.
I disagreed pretty strongly with some of MacGregor's interpretations (he still writes within the weird idea that there is such a thing as "primitive art" as a generic category), but this thing was so thorough and dense, it's essential for anyone interested in the topic. It's not particularly exciting for the most part and the need to constantly reference images as you read (and how much this slows down the reading process) will be familiar for readers of art history. The writing often takes on the tone and style of encyclopedic entries. I would recommend tackling chapters individually as they come up in research rather than read cover to cover as I did.