Night soldiers (1988)

Beginning in 1934 Bulgaria, Khristo Stoianev's recruitment into Soviet intelligence leads through civil war, purges and occupied Europe. This is a broad historical journey with danger and action, but its defining question is how a person survives the changing demands of supposed allies. The first of Furst's loosely connected espionage novels, it can be read on its own. Especially rewarding if you want several political borders to change the meaning of loyalty.