Macroeconomic textbooks

From Brad DeLong

There is an apocryphal rule about new textbooks: they can only have 15% new material. A successful new textbook must be different enough from the old standards to give professors an incentive to switch, but must to similar enough to the old standards to keep the process of switching from requiring professors to throw away all their old lecture notes and completely redesign their courses.

This is a neat trick. It makes intellectual progress--at least intellectual progress in undergraduate instruction--nearly impossible.