The letter, from Crown lawyer Aedeen Boadita-Cormican, said matters referred to in the book "lead us to believe that email communications with the plaintiffs ... contain information relating to the events which are the subject of these proceedings".
It went on to say: "We seek copies of the emails and/or records of interviews referred to and we would be grateful if you would make these available for inspection by us or provide copies to us."
The move has surprised a legal expert and the book's publisher, who both believe it is unusual for a writer to be asked to give up notes.
Paul Little, who published The Secret Life of Kim Dotcom, said: "I've never seen anything like it as a writer or editor before. We're taking (legal) advice and once we've got that advice we will know where we are going from here."
The whole thing was "mystifying - but everything around this story is mystifying".
Professor Jeremy Finn, of the University of Canterbury, said Crown Law could ask for the information but would not necessarily receive it.
"The first question is, is it common having police seeking information of this kind? They frequently do seek material in the hands of third parties, though this is the first time I have heard of it being (requested of) a biographical author."
The second question would be on what basis Crown Law was seeking the information, which was not clear. "It seems to be a fishing expedition."
The letter said if Fisher was "unable or unwilling" to provide the information, Crown Law reserved the right to pursue the matter in court.
Where it stands
• Kim Dotcom is seeking $6 million compensation from the police and the GCSB over the unlawful raid on his mansion and illegal spying.
• He launched the civil case in the High Court at Auckland in September.
• In November, journalist David Fisher and publisher Paul Little released a book about Dotcom.
• Police and the GCSB are now seeking information gained by Fisher while he researched the book.
