Amazon, no doubt.
Actually, I've just acted on your tip, and ordered two books from Black Front Press: Southgate's writing on politics and history, and one on 'counter-culture; music, film, and literature'. The latter apparently includes interviews w/ Turbund Sturmwerk, Puissance, Endura, and Ostara, as well as reviews of industrial, experimental, neo-folk, metal, and gothic records and concerts.
Apart from a few tracks here and there, I was always a little disappointed by Southgate's musical output. We'll see what he has to say for himself, anyway.