No prizes for guessing which author dominates the Finnish lists on both LibraryThing and Goodreads. Famous for her children's fantasy novels about anthropomorphic creatures living almost human lives (apart from hibernation), I've been very glad to discover some of her work for adults which has been getting into English translation in recent years.
The Goodreads winner is a bit surprising, though; it was the first of the children's books to be published in Finnish, in 1945, but the last to be translated into English, in 2005, and reviews suggest that the author had not yet hit her stride. I devoured all the other books in the series as a kid, but haven't read this one. It is:
The Moomins and the Great Flood / Småtrollen och den stora översvämningen, by Tove Jansson
The LibraryThing winner is the one I would have expected, first published in 1948 and the first of the series to be translated to English, in 1950, in which our heroes have a series of loosely-connected adventures, several of which centre around a magic hat. It's fourth on Goodreads (whereas the previously-mentioned Goodreads winner is way behind on LibraryThing), so I am declaring it today's winner. It is:
Finn Family Moomintroll / The Happy Moomins / Trollkarlens hatt, by Tove Jansson
The second-placed book on GoodReads, third on LibraryThing, is a lovely short novel for adults, by this same author, about the relationship between a grandmother and granddaughter spending a summer on one of the Finnish islands. Originally published in 1972, translated into English only in 2003, it is:
The Summer Book / Sommarboken, by Tove Jansson
That is just behind the second-placed book on LibraryThing but well ahead of the third-place on Goodreads, Finland's national epic, compiled (composed?) during the nineteenth century and published in 1835, an expanded version appearing in 1849. In 50 chapters of trochaic tetrameter, it is:
The Kalevala, by Elias Lönnrot
That's the highest-placed book on the list originally written in Finnish (the others were originally written in Swedish).