Hi Oak,
Two books come to mind, but I don't know if they are available as new anymore. Perhaps you can find good used copies. I think these apply to the area you are interested in.
BAHNSTORMER
The Story of BMW Motorcycles by L.J.K. Setright (Yes, that is his real name)
covers the mechanical changes of the bikes, some of the racing history
ILLUSTRATED BMW MOTORCYCLE BUYER'S GUIDE
by Stefan Knittel and Roland Slabon
covers mostly the the 1960's to the mid 1980's, some of the 1950's
Good luck,
~ ~ ~ jaknight ~ ~ ~