http://www.autogallery.org.ru/mbmw.htm
Very nice photos and documents for pre-war BMW motorcycles.
http://www.fndiffusion.ch/
Extensive source of available lists and manuals.
http://www.themotorbookstore.com/
Online book seller specializing in motorcycle books.
http://www.merlinbooks.com/
Online bookstore.
http://www.Zweirad-Literatur.de/
Information and manuals for vintage motorcycles. German only; English link doesn't work.
http://www.motorbooks.com/
Large catalog of motorcycle books.
http://histor.ws/dmag/index.htm
Specifications and designs for 1930s era magnetos.
http://www.automotive-tradition.de/en/wissen/recherche.htm
Archives for Bosch electrical systems.
http://www.realoem.com/bmw/select.do
RealOEM has on-line fiche for BMWs.
http://www.histor.ws/bmwr12/eng/index.htm
Information on the history, design, and restoration of the R12.
