Author Elisabeth Asbrink says Mr Kamprad was an active recruiter for a Swedish Nazi group, and stayed close to sympathisers well after World War II.
The details go beyond what Mr Kamprad has previously admitted.
The Swedish billionaire has said his involvement was youthful “stupidity”, and the “greatest mistake” of his life.
In her book, Ms Asbrink says Mr Kamprad actively recruited people to the fascist Sweden’s Socialist Union (SSS).
She says the activity prompted security police to set up a file on him in 1943 when he was 17 – the same year that he founded Ikea.
Ms Asbrink says the security police intercepted his post, and noted that he “had some sort of functionary position” in a youth Nazi organization.
Phantom Report: Read: The Nazis go underground by Curt Riess. The book is out of print. Here is the pdf format: Nazis go underground