This is a self-published book by Ivan Kolev, a Bulgarian auto historian, painter, journalist and massive car fan.
It was published in 2010, hard-cover, 410 pages (!!!)
It is a year-by-year overview of Bulgarian motoring history, including cars, trucks and buses. Local coachbuilders, dealers are features, also stories on individual cars which were imported to Bulgaria. An incredible amount of information has been gathered together.
Only drawback is that it is in Bulgarian.
Price is EUR 35 including p&p. If anyone is interested, I can relay requests to Ivan