It's history. The Soviet Union can be treated as a finished historical entity. The time has arrived to separate wheat from chaff and identify the features, flaws, and moments that mattered. Keep, a distinguished British historian, does that in a well-rounded account designed for a general audience. He starts with the Stalinist order, traces fumbling efforts at change under Khrushchev, and describes the stirrings of civil society in the stagnant Brezhnev era. A good third of the book is devoted to the Gorbachev period. Keep reviews not only Gorbachev's initiatives -- the "revolution from above" -- but also the gathering stream of social restlessness and nationalist sentiment into which they flowed, producing the more powerful "revolution from below." In a story like this, however, the ultimate point of the history must be to explain the denouement. Keep not only refrains from judging the Soviet Union's ultimate legacy but, in a brief afterword, also passes quickly over the reasons things turned out as they did -- a pity because these few paragraphs contain hints of promising ideas.