Finland Historical Geography

The borders of Finland have changed several times, beginning in 1323 with the Treaty of Nöteborg between Sweden and the Novgorod Republic, through 1947 after the end of WWII between Russia and Finland. Finland's Ministry for Foreign Affairs has published Tracing Finland's Easter Border which illustrates the many changes to the border with Russia.

There are two very good articles on Wikipedia discussing the cession of Karelia to Russia.
 * Karelian Question
 * Finland-Russia Border

The National Land Survey of Finland has published a brief recounting the documenting of the boundary demarcation on the border between Finland and Russia which took place on 19 June 2017.

The Family History Library in Salt Lake City, Utah has the following book that describes the history of the borders of Lappland in Norway, Finland, Sweden and the changes that occurred from 1751 to 1944. It is written in Norwegian and can be checked out at the Access Services Window on the International Floor of the Library.

Norsk folkemuseum. Grenser i Sameland. Oslo: Norsk Folkemuseum, c1989.

https://familysearch.org/search/catalog/565500?availability=Family%20History%20Library

Historical Provinces and Counties
Additional maps showing the changes in county boundaries can be found at Suomen läänit (in Finnish).