Why did the US support the Khmer Rouge and what's the best book available on this issue?

I'm backpacking through Southeast Asia and today I visited Cambodia's Killing Fields and the Genocide Museum. I haven't been able to study the Khmer Rouge, but I was interested in some of the writings in the museums--one display said that the US and its allies gave substantial assistance to the Khmer Rouge, but other displays said that Khmer Rouge officials killed anyone suspected of having any involvement with the CIA, which seems inconsistent at least superficially. I read a display that suggested that the US supported the Khmer Rouge only because they wanted to help any enemy of Vietnam, but is that all there was to it or was there more going on? Can anyone suggest a good book on this topic? (And while I understand that the human side is very important and terrible, I'm interested in reading a book that more discusses the political/international relations side of who supported the Khmer Rouge and why.)