Dmitri Volkogonov - Autopsy For An Empire, seven biographies of the leaders of the USSR. Some of the key historical events are not covered in detail, but its quite penetrative on the failings of the regime and the leaders. Contains lots of anecdotes, as it's from an ex-military general no less. I found the bits on Andropov and Chernenko particularly useful.
Vladislav Zubok - Collapse: The Fall of the Soviet Union, released this year. Vis a vis Volkogonov, dwells more on the emergence of various nationalisms and the contradictions in Gorbachev's social policy.