AFnord: Finding specific information about Finland during the middle ages can be quite tricky. As Finland was a part of Sweden during most of the middle ages (starting from the middle of the 12th century, and fully "conquered" during the mid 13th century) your best bet is probably to try to find books about the history of Sweden, and hope that they give Finland the time it deserves.
There is
very little to go on from times before the Swedish conquest (mid-12th C.), as there are no writings from before that time, only a passing mention of the
Fenni in Tacitus and a few runes from Sweden mentioning
finlont or
finlandi. After that, pretty much everything is from the viewpoint of the Swedes, until the beginning of Finnish nationalism.
Finnish oral tradition, much of which was collected in the
Kalevala and other works, is a much richer source than the scant written history. Although its historicity is shaky, its influence on music and literature has been tremendous. You may be particularly interested in the works of sci-fi writer Emil Petaja, who drew heavily on the
Kalevala.