As mentioned above, there is no such thing as unbiased, however if you're looking for books on north Korea that aren't exaggerated lies and hyperboles spread by the west, here are some of my suggestions based off what I know (Note that I haven't read all of them):
Any of the hundreds of books from korean-books.com.kp/en
Facts Tell Secret Documents Seized by North Korea from South Korean Government Archives https://annas-archive.org/md5/8a0e6e72f1fc456d78943f6bd13b4674
In North Korea: First Eye-Witness Report by Anna Louise Strong https://www.redstarpublishers.org/ALSKorea.pdf
Korea Is One https://annas-archive.org/md5/4b4a58dc5be40d5986d061a9b60a736c
Modern History of Korea Published by Foreign Language Publishing House (I can't find this online currently, but I can send you a PDF if you ask)
Out of Their Own Mouths: Relevations and Confessions Written by American Soliders of Torture, Rape, Arson, and Cold-Blooded Murder of Defenceless Civilians and Prisoners of War in Korea (I can't find this online currently, but I can send you a PDF if you ask)
Shall Brothers Be... An Account Written by American and British Prisoners of War, of Their Treatment in P.O.W. Camps in Korea https://ia801601.us.archive.org/28/items/shall-brothers-be/Shall%20Brothers%20Be_text.pdf