G. Mackenzie Bacon - On the Writing of the Insane (1870)
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Don't get too excited that this is an early discovery of outsider art (Adolf Wölfli would have been six when this appeared) or rather it is but the author didn't quite make the appropriate connections. It's also a brief pamphlet with nearly all the illustrations excerpted below and the text not being much of an anthology. Still, it's free, short and fairly interesting.