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Dieters bok is based primarily on three source groups: Karin Bergman's diaries and ”thought books”, Dieter's diaries and the author's interviews with those involved, mainly with Dieter himself.
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A large part of the other source material consists of the letter collections in the Calwagen-Bergman family archives in the National Archives, Riksarkivet.
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Among other things, there is the extensive correspondence within the family, including Dieter's letters to Karin and Erik Bergman in the 1950s and until Karin’s death in 1966.
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Karin Bergman's ”thought books” have sometimes been called her ”secret diary”, but that's not a correct description. The ”thought books” are a couple of exercise books, filled with non-periodical notes, not always dated. One can find letter transcripts and thoughts about religion but also about family members.
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One of the most unique experiences in the work with this book has been to study the same chain of events depicted closely by the two protagonists of the book - Karin and Dieter - in their respective diaries.