One of the many fun things about Greg Mankiw's blog -- besides the passive-aggressive spats with Paul Krugman -- is that he has an admirably shameless way of shilling for his own textbooks. (See, for example, here, here, here, here, here, here and here -- all pulled very quickly and more or less randomly from the past two months.)
I used those textbooks in college and think they're pretty great. But when I read that Tyler Cowen and Alex Tabarrok are releasing their own Macroeconomic Principles textbook I was reminded of an old post from Mankiw:
[...] It is true that my university salary is reasonably secure, but more of my income comes from book royalties than salary, and that income is anything but stable. Any day now, someone could come along with a better textbook and put me out of business.
On this last point, of course, I am speaking hypothetically.
More competition, as Mankiw would say.