Novel sales figures

Not really. There are venues (such as Nielson Bookscan) that tell you certain figures through certain channels, but the fact is there are so many different distribution channels... not even the publishers know all the figures, all the time. And if you're talking about classics such as To Kill a Mockingbird, it's basically impossible. There are so many versions, so many different publishers, so many different channels. I don't even think the author knows.

You can get indicative figures for a book first published in the last few years, but even those can miss huge chunks of sales.

What can be chalked up if you're an insider is the number printed, which is why Nora Roberts or James Patterson will sometimes have a McDonald's-type Over Forty Trillion In Print. That is, companies know what their print run was, and you can track those...

...if you're an insider. But companies also sometimes lie about their print runs, usually exaggerating them upwards. But return rates from the stores have hit 70% on some books in the last few years, so printing and sales only have a vague relationship. (The only thing you can be sure of is that sales didn't exceed print runs.)

But for widely reprinted classics--like the estimable Maestro indicated, you can forget about it.