I know about this very well. I studied East Asian history at university. There are many English language non-fiction books about the Rape of Nanking.
A very popular American novelist, Martin Cruz Smith, write a bestseller called Tokyo Station. This book has a key and quite horrible scene set during the Japanese atrocities against Chinese citizens.
There is no reason to disbelieve the many accounts of the atrocity. The Imperial Japanese Army was formidable, seasoned in combat, and at the beginning of the Pacific War had weaponry much more advanced than other countries including the US. It had what English speakers call “grit” - a relentless desire for success, and remarkable patriotism. And it had gifted leaders like Yamamoto. Only through a lack of oil and some extremely bad luck (the Battle of Midway, and the absence of the carrier fleet during the air strike on Pearl Harbor) did it lose the war. The Chinese armies (both PLA and KMT) were mostly made up of very poorly equipped and inexperienced peasants. There is no reason at all to think that the Chinese somehow allowed the Rape of Nanking to happen.
(My old Japanese tutor, who was from Kobe, was aware of Japanese atrocities during the war but said she only became aware of them when she studied history at university.)