While this is not a book, it is an article and available here for those interested: http://sovietinfo.tripod.com/WCR-Secret_Police.pdf
The article is mainly concerned with the ongoing debate, at the time, between Wheatcroft and Conquest, amongst others. Robert Conquest is a part of the older generation who wrote about the Soviet Union in the midst of the Cold War. His "The Great Terror" written in the late 1960's painted the Soviet Union in a rather deathly light. His sources were for the most part emigres and resentful defectors. Please, keep in mind that what these people had to say was important but only to a certain degree. What Conquest does is parade their ideas and stories to correspond to everything that was going on in the Soviet Union when in fact such generalizations are ignorant at worst and guess work at best.
Today, the archives in the Soviet Union have been open for over a decade and a plethora of research has been ongoing into what it really meant to live in the Soviet Union and what was Stalin responsible for when it came to the purges, famine(s), etc. The above mentioned article presents some of the archival data that come to light recently and juxtaposes it with Conquest's data which even after witnessing it he still tries to deny or pass off as falsified data. Wheatcroft takes the time to explain why Conquest's ideas are baseless and why the new research has to be taken into consideration in order to draw a coherent understanding of what went on in the Soviet Union during mainly the 1930's and put it into context with the rest of Stalin's tenure at the head of the USSR.
This is very dense and detailed information but taking the time to read it will benefit the reader and help him/her understand why cold war literature is just that, literature and to a large degree not studies that need to be read today in order to have a viable opinion and understanding of what went on in the Soviet Union during the purges. The famine of 1932/33 is addressed at the end of the article only because Conquest makes mistakes in his data and information regarding what happened and why it happened in his book "The Harvest of Sorrow."
This article is a good introduction into the GULag system and in regards to the Soviet Secret Police. Today there are other books that have come out which continue to shed new light onto this time period and the events in question.