But he made some lucky finds in libraries in northeastern China, too, while also extracting papers from reluctant librarians at China’s major archives. And through his connections with top party officials, he secured access to a memorandum of conversations between Mao and the North Korean leader, a trove that had never before been made public.

In the book, Mr. Shen demolishes the myth that China and North Korea were tightly allied, “as close as lips to teeth,” as China’s propagandists insisted. He shows that even before the start of the Korean War in 1950, relations between the two newly installed Communist parties were tense. Mr. Kim, the grandfather of the current North Korean leader, Kim Jong-un, invaded South Korea without notifying Mao. The Chinese were informed three days after the fact.

Despite his current pursuits, Mr. Shen is hardly the scholarly sort who has spent his life buried in the stacks. He started out as a crack Navy pilot, but that ended when he was falsely accused of murder by a jailhouse snitch. Mr. Shen was freed after the informer recanted, but was jailed again in the early 1980s on an accusation of spying for the United States.

Mr. Shen says the spying charge stemmed from his giving some articles and documents on Chinese agrarian reform to an American student who, unbeknown to him, was suspected by the authorities of having links to the C.I.A.

During his two years in prison, he invented a way to write by fashioning an empty toothpaste tube into a pen. He asked for books on Mao, Marx and Lenin (“The prison authorities didn’t dare deny that”), and wrote notes for his first work — on Soviet agriculture — in the margins of the Soviet Union’s “New Economic Policy,” using an upturned washbowl balanced on his knees as a desk.