What is the best book about each country?

For Finland it's probably either Under the North Star (Täällä Pohjantähden alla) or The Unknown Soldier (Tuntematon Sotilas).

I don't think Sotaromaani, which is an uncensored version of The Unknown Soldier, has been translated to English.

https://en.wikipedia.org/wiki/Under_the_North_Star_trilogy https://en.wikipedia.org/wiki/The_Unknown_Soldier_(novel)