If I'm writing a book on a subject, I'm going to write it because I understand it well and think I have something interesting to say. I won't just pick something that I think makes money and then bullshit something into profitability.

Come to think of it, that's also how I look for the good startup companies amidst all the bad: When you find the ones run by people who understand what they're doing, you find a satisfaction in their output that you don't find elsewhere. 280North comes to mind, as does Contrast.io.