I have recently been researching on East Asian communism and the socio-political impact experienced in the region because of it. I have been looking at a good number of sources on Cambodia especifically, and many of these tend to point the work of Ben Kiernan, Yale-based historian and director of the Cambodian Genocide Program of the same institution. I have read some criticism on Kiernan's old views and very early statements where he downplayed the abuses and crimes inflicted on Cambodians at the time. He changed his views a long time ago and many consider him a lead scholar on the subject. Still, I remain cautious and would like to read a second opinion on the subject from someone better versed on it.
Thanks, /u/commiespaceinvader. I understand why you're apprehensive, but Kiernan is widely regarded as a good scholar and his book on the Khmer Rouge is the primary text on the subject. As /u/clavdivsimp notes, Kiernan studied under the major Cambodia scholar David Chandler, was open about his mistake, and then spent his career trying to shed light on the Khmer Rouge, genocide, and to help bring the KR leadership before trial.
The only critique that I've heard of the book is that too much emphasis was given to the US bombing campaign as giving rise to the KR's popularity -- Kiernan's argument being that the destruction of the bombing was exploited by the KR to recruit, thus giving rise to Pol Pot's organization which was previously very weak.
While Kiernan certainly has proof to say that was one component, others note that not enough attention was given to how North Vietnamese and the southern PLAF/Viet Cong used Cambodian territory as a haven. And when US and South Vietnamese forces raided these border areas, these troops pushed further into Cambodia. This enflamed the historical enmity between Khmer and Viets, and likely was equally or perhaps more direct in leading to the growth of the Khmer Rouge. The Khmer Rouge would supplant the 'pro-Vietnamese' Khmer revolutionaries that trained in North Vietnam and were subordinate to the Northern Vietnamese leadership.
In direct contrast, the KR used a fiercely anti-Vietnamese rhetoric to recruit. They would go on to slaughter the Vietnamese population in Cambodia after 1975, and thereafter made cross-border raids on Vietnamese populations. Unfortunately the scholar who is working on the break between the Khmer Rouge and Vietnamese communists is still finishing the project, so I don't yet have a citation to give you.
paging /u/Lich-Su for their excellent knowledge on the subject