The lighthouse island's coordinates in "Pappan och Havet" (as explicitly specified on Tove's own drawing for the book's title page) are:
Lat: 60°7'12" N Long: 25°45'50" E
Or in decimal form:
Lat: 60.1200 N Long: 25.7639 E
Popular Web-based maps (MapQuest etc.) show no land at these coordinates. However, given the precision of the information on Tove's drawing, I somehow fail to believe that the island was just a product of her imagination. I guess there are a few people among my friends who have access to good navigation maps of the Finnish Gulf (old_skipper?). Could you please check if there are any traces of land at or near this location? A rock, maybe. Or, hopefully, a real lighthouse.