This is a somewhat enlightening biography of Pol Pot, and therefore a selective account of the political situation in Cambodia during the 20th century.
One has to take a shine to a book that asks the question you're interested in on the third page; in this instance, just what the hell did the Khmer Rouge have in mind? How could the leaders of a country decide to decimate it so thoroughly?
Ultimately the book fails to provide a satisfying answer, but does justify this failure by showing how thin the record is. I came away with the impression that the party was a dictatorship of one man who managed to play his underlings off against one another with sufficient skill to remain in the role of chairman almost to his death. It is perhaps most difficult to comprehend why the fellow travellers went so far with him in the face of such thorough-going and brutal purges.
Politically Brother Number One seemed to think that the individual's only worth was in the labour he or she could provide to the state. With most of the expertise of returning Cambodian ex-pats squandered (they got executed), the regime was always heavily dependent on foreigners for anything more sophisticated than the most primitive agricultural techniques. Apparently there was no contradiction here with the idea that Cambodia is (in Western speak) God's own country, and neither is there one with the party leadership coterie living in relative comfort while their countrymen endure enforced poverty.
Most shocking is the incompetency of the Democratic Kampuchea regime, and the realpolitik machinations of a United States that had just begun to come to terms with their conflict with Vietnam. Pot entertained some pretty weird ideas of being rescued by the U.S. military, though he was right to bank on some support against the newly re-unified and communist Vietnam. In the end it was Vietnam, through occupying Cambodia in 1979, that sorted this particular mess out for the people of Cambodia. The occupation lasted about ten years, and so it is for only a relatively short time (almost twenty years now) that this country has been at peace.
I have no idea what the current regime is or how they reconciled the border tensions with Thailand. (Clearly the new government is friendly enough with Vietnam.)
There are some thoughtful reviews at Amazon. I expect one of the more recent biographies would be even more insightful.