Experience shows that a few years into such a new movement, it is often helpful if someone provides a synthesis-typically a book that shows how many seemingly disparate models can be viewed as varia- tions on a few main themes, that develops a common ”grammar” for discussing a range of issues. Such books as Helpman and Krugman 1985, on imperfect competition and international trade, or Gross- man and Helpman 1991, on endogenous growth, helped give shape and direction to the new fields they surveyed