This period is definitely the most popular one to write about, so there are many books. (Seriously, I'm wondering how you didn't find anything!) First of all, I would recommend A History of Cambodia, by David Chandler, which covers all of Cambodian history including the period before and during (but not after) Democratic Kampuchea. Then there's How Pol Pot Came to Power, by Ben Kiernan, which covers the details of how both the Pol Pot faction came to control the Cambodian Communist movement, and how they came to control the entire country. I would also recommend Cambodia 1975-1982, by Michael Vickery, which gives some very good context to a period that is often described with a lot of hyperbole. For the downfall, the best book is probably Brother Enemy by Nayan Chanda.

Once you've read those, there are a couple more biographical books, covering the two most important leaders of Cambodia, who bookended Democratic Kampuchea: Sihanouk: Prince of Light, Prince of Darkness, by Milton Osbourne, and Hun Sen: Strongman of Cambodia, by Harish and Julie Mehta. These two interesting books fill in a lot of gaps. There are a few biographies of Pol Pot himself, but I honestly can't recommend the ones I've read - there doesn't seem to be a lot to say about him. Hun Sen, Norodom Sihanouk, or even Khieu Samphan (President during Democratic Kampuchea; Pol Pot was Prime Minister) were all more interesting (and better documented!) figures.