Robert E. Lerner 
Heresy of the Free Spirit in the Later Middle Ages, The [PDF ebook] 

The Heresy of the Free Spirit in the Later Middle Ages has been widely recognized as the standard work on the subject in any language. Robert E. Lerner examines this fourteenth-century European heresy as it appeared in its own age. He concludes that the Free-Spirit movement was not a tightly organized sect of anarchistic deviants, but rather a spectrum of belief that emphasized voluntary poverty and quietist mysticism.

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