How's your German? There's a recently published doctoral thesis by Dorothee Hochstetter called 'Motorisierung und "Volksgemeinschaft": das Nationalsozialistische Kraftfahrkorps (NSKK) 1931-1945.' Publisher is Oldenbourg Wissenschaftsverlag, Munich.
You may also come across a book called 'Driving Forces - the Grand Prix World caught in the Maelstrom of the Third Reich' by Peter Stevenson. Treat this with EXTREME caution, since the author, while growing up in the USA in the 1950s, seems to have come to his own conclusions about the subject and, when he eventually wrote the book, has sometimes fitted the facts to his theories rather than the other way round.
I hope the mods won't mind, but we have had extensive discussions about this subject at the Atlas Nostalgia Forum BB: here's a link to just one of the many threads there.
http://forums.autosport.com/showthre...376post2207376
There's also much of interest at the 8W site:
http://8w.forix.com/index.html