I know the story about Seija Mattsson from Ola Tungesvik whom I met in Jan.2003 in Tenerife where I spent my vacation. I tried to find English versions of all books about Seija including Ola"s book but I could not.
Ola Tungesviks book about Seija Mattsson was originally written in Norwegian ( Om jeg får vaere lykkelig…). The book has been translated to Finnish by the name Jos vain saan olla onnellinen…: tosi kertomus rakkaudesta by Tarja Teva 1999. Finnish Swede writer Gunnar Mattsson has also written about his wife Seija Mattsson (Prinsessan, 1965), and that book has been translated to English by the name The Princess.