One million copies is a lot of anything, but should it influence your decision to buy a book?
No it shouldn’t. Judging from the things that other people wear and watch and the ways they spend their time, a million people doing something should have no influence on whether I do it too. At best, they have totally different taste. At worst, they are utterly wrong.
For things that people with similar taste to me might buy, though, there’s something in it. Successful marketing campaigns add noise to the signal, but if “people who like this kind of thing” liked this one enough to buy a million copies, it’s probably worth checking out.
The right response…
… when someone tells you that a million people have bought something is probably to ask: “Which million?”