Without knowing... precisely the kind of content you are after I could probably recommend a few?

• Ben Kiernan's How Pol Pot Came to Power: Colonialism, Nationalism, and Communism in Cambodia 1930-1975. It is a very thorough study, foundational even, for the lead up to the Khmer Rouge taking power. A bit of a dry read, but certainly lots of detail regarding Chinese relations before the KR period... without it being a major focus however. His other 'big' book on the subject, dealing with the regime's time in power might also be appropriate: The Pol Pot Regime.
• Philip Short's biography of the Khmer Rouge leader, Pol Pot: The History of a Nightmare, might also be worth your time. Short was a correspondent in Beijing during the Khmer Rouge's time in power and his book about Pol Pot includes a lot of information dealing with the Chinese relationship before, during and after the KR period.

Its a pretty niche topic that you are after I guess and I can't say I know much about that particular area off the top of my head apart from fairly broad strokes.

You might be interested in Sophie Richardson's China, Cambodia, and the Five Principles of Peaceful Coexistence (2010)? It should cover the time period you're looking at. I haven't read it myself, but the reviews by scholars of Sino-Vietnamese relations seem to be favourable.

I realise this is about two months old but I recently ordered Brothers in Arms: Chinese Aid to the Khmer Rouge, 1975-1979 by Andrew C. Mertha, can't say how good it may be but maybe you would be interested