Book Reviews : The Soviet Block Unity and Conflict. Zbigniew Brzezinski. Harvard University Press. London. Oxford University Press. 62s. The Foreign Policy of the Soviet Union. Edited by Alvin Z. Rubinstein. New York. Random House, $6.60. The Challenge of Co-existence. Milton Kooner. Public Affairs Press. Washington. $3.25. The Soviet Economy. Alec Nove. Allen and Unwin. Cloth at 25s. Paper 15s