Kevin Drum is nonplussed by something he read in the WSJ: a Canadian named James Stewart who got freakishly rich by writing calculus textbooks. As Kevin says, "You can earn millions writing calculus textbooks? Seriously?"
Well, this isn't much of a stretch, is it? Here's the guy's website -- he has written a number of texts, which seem to have each gone to multiple editions, which implies that his textbooks have been adopted by more than a few math programs around. Calculus is, I assume, one of the more frequently taken courses in colleges throughout North America. Many kids take Calc I just to get their math requirement out of the way, and many, if not most, of the science majors require Calc as part of the major. And you can probably throw in high schools that offer their own Calc course for seniors.
So if this guy has written a text that gets used in, oh, just ten percent of the undergraduate programs in the US and Canada, that's a hell of a lot of copies of his book being moved every year. And I'll bet his text gets used a lot more than that. One of Kevin's commenters notes that Stewart's book is used at Penn State University. That's a big campus, right? I'll bet the number of kids who have to buy that book every year at Penn State is in the hundreds. And I remember how awfully expensive textbooks were when I was in college, nearly twenty years ago; I wonder how much they are now? Shudder!
Maybe this doesn't seem at first glance like a field one should be able to get filthy rich in, and one probably can't these days, since Stewart may be dominating the market with his books. But this certainly seems to fall into the category of "Nice work if you can get it."