I'm very interested in these matters, and I' like to find a good book about it (in english). Especially the relationship between the "Kalevala" and the real mythology, the ideas produced by Lönnrot that not to be in the folk tales, etc.
I've found in the web this book: "Kalevala Mythology. Folklore Studies in Translation", by Juha Y. Pentikäinen. Translated and edited by Ritva Poom. Indiana University Press, 1999. It seems to be perfect for my wishes, but I wonder if anybody knows another better than this. Tell me, please!
I'm sorry, my english isn't quite good! My blog about Sibelius in spanish: