Elhanan Helpman & Paul Krugman 
Market Structure and Foreign Trade [PDF ebook] 
Increasing Returns, Imperfect Competition, and the International Economy

Market Structure and Foreign Trade presents a coherent theory of trade in the presence of market structures other than perfect competition. The theory it develops explains trade patterns, especially of industrial countries, and provides an integration between trade and the role of multinational enterprises.Relating current theoretical work to the main body of trade theory, Helpman and Krugman review and restate known results and also offer entirely new material on contestable markets, oligopolies, welfare, and multinational corporations, and new insights on external economies, intermediate inputs, and trade composition.