Kalevala. Elias Lönnrot collected traditional poems into national epic. Nowadays there are huge amount of streets, suburbs and companies in Finland named after Kalevala. Nerds, attention: J.R. Tolkien was inspired by Kalevala, and the outcome of his inspiration was the Lord of the Rings.
Plot in a nutshell: An old guy falls in love with a young girl. A younger guy has also his eyes on the girl. The old guy sings the young guy into swamp. A blacksmith makes a money-making machine. A bitch from north tries to steal the machine, and the old guy and his folk try to protect the machine.
Seitsemän veljestä - Seven brothers. Written by Aleksis Kivi. Kick-off for realism in the Finnish literature. Also a kick-off for respecting Finnish language within Finnish literature (until this the Finnish writers were writing mostly in Swedish).
Plot in a nutshell: Seven brothers are trying to learn to read. The youngest of the brothers starts to read first. Heard of bulls are chasing the brothers, the brothers survive barely by climbing on top of a stone. Some of the brothers get married.
Tuntematon sotilas - Unknown soldier. A mediocre war novel about Finnish battles in WW2 written by Väinö Linna. The whole story relies heavily on the stereotypes of Finnish tribes and dialects. Due to the dialects the English translation just doesn't work.
Plot in a nutshell: Finnish men are called into duty in summer 1941. The war starts. Some die. Some are not taking orders. Finns are fighting persistently against the giant foe, but still they loose the war.
Nuorena nukkunut - The maid Silja. Written by F.E. Sillanpää, who's the only Finnish writer awarded with Nobel prize in literature.
Plot in a nutshell: There is a servant girl named Silja. She gets laid once and misses the guy until her last breath. I've read the novel twice, but I can't remember anything particular about it. Which probably tells something.
If you're planning to tackle a classic piece of Finnish literature - especially translated into English - I can't recommend any of the classics listed above, to be perfectly honest. They are meaningful stories and they have their place in the history, but they may not be the nicest ones to start with. If you want to read fine Finnish novels translated into English, I'd recommend you to grab on The Egyptian by Mika Waltari or Purge by Sofi Oksanen. I'd like to recommend also Vadelmavenepakolainen ("raspberry boat refugee") by Miika Nousiainen, but unfortunately it's not translated to English (yet).