A few links to documents relating to the Group d’Information sur les Prisons and a question about further material. Here’s what I know –

Marcelo Hoffman’s Foucault and Power has a translation of the ‘Inquiry into Twenty Prisons’ as an appendix; and there is a collection of documents on and by the GIP, including from the Intolérable collection and the earlier (and out-of-print) IMEC volume Archives d’une lutte, forthcoming from University of Minnesota Press.

Update: Marcelo has pointed out there is also a translation of ‘The Masked Assassination of George Jackson’ in Joy James’s Warfare in the American Homeland. There is a pdf of it here.