I have read part of Leszek Kolakowski's book Main Currents of Marxism: The Founders-The Goden Age- The Breakdown and agree with those who find it a wonderfully informative book. Kolakowski was a Marxist in his youth and became the Chairman of the Department of Philosophy at the University of Warasaw after WWII. His book shows why Marxism is appealing as well as showing insight into the historical problems of implementation. There is recently a nice obituary of the author in the Financial Times; Christopher Hitchen's also comented in Slate.