Kim Jong-nam fell out of favour years ago and has been living in China.
Japanese author Yoji Gomi said he was no longer in contact with Kim Jong-nam but believed he was “doing fine”.
Mr Gomi told the BBC that he did not believe that the publication of his book in January 2012 would put Kim Jong-nam in any danger.
When his book was published, readers gained an insight into North Korea’s enigmatic first family through the reported views of Kim Jong-nam.
He was the man passed over in the world’s only Communist dynastic succession.
Kim Jong-nam’s opinions about North Korea’s future course after the death of his father Kim Jong-il were far from complimentary.
In the book, which draws on interviews and emails, Kim Jong-nam is quoted as saying he believes his younger half-brother lacks leadership qualities, the succession will not work, and that North Korea is unstable and needs Chinese-style economic reform.