In terms of character arch and what each of the Leo Demidov/Child 44 books contribute, Agent 6 ends the series nicely. I will say at the outset that this particular book could have been two separate stories but more on that later.
Each book is a symbol for the Soviet regime in which it takes place. Child 44 (the first in the series) represented the harsh, oppressive, gritty, ruthless years of Stalin. The Secret Speech finds Leo (and the Soviet Union by extension) confused and conflicted in what to do with their harsh, shameful past and a desire to move forward differently but ill-equipped to do so – much like the Khrushchev years. Agent 6 is set during the Brezhnev years which were marked with ill-conceived wars with countries like Afghanistan, a crumbling economy, and communism in a general state of disrepair being led by people who refuse to see this and continue to make ridiculous self-destructive decisions in spite of the facts. True to form, Leo Demidov embodies these same realities.
The previous two books relied on equal parts mystery and thriller. Agent 6 plays heavy with politics and character development. Smith gets lost in the politics and propaganda of both sides, expertly weaving in key historical dates into the narrative and delivers a cutting critique of both free market capitalism and communism. In so doing, Smith reveals it’s the darkness of the human soul that ruins the world, not so much a particular system. There is something inanely wrong with humanity.