As the Fletcher Hanks collections show this group is often willing to purchase the books even after having the scans. Essentially the scans are free advertisements.
I don't know if I agree with that, but that's more because I lack any real data on the subject than because I've given the topic serious thought or analysis. And, of course, I bring it up in case somebody has better information than I do.
What I do know is that (when there's surplus money on hand) I'm personally likely to buy products that I find interesting, regardless of whether I already own the equivalent material. But that's more moral support and an interest in seeing MORE interesting products (that I might really want) than because I was "sold" by downloading something for free. But, as I said, I don't know how common that idea is versus any other line of reasoning.
The only people that I can think of who might have worked out the details are folks like (author) Cory Doctorow and (songwriter/musician) Jonathan Coulton...but their data would probably be skewed, since they're well known for railing against big corporate copyrights and attract many fans from those demographics. And Coulton recently posted a message on his blog that pretty much says that he doesn't know. I seem to recall that Doctorow's opinion was something like his publisher is still signing him, so it can't be too much damage...but that's hardly scientific.
Regardless, though, copyright means that it's not our right to tell them how to distribute their work, whether we think it'll help or not. I'm pretty sure, for example, that you could get into pretty deep trouble for running an advertisement in your local newspaper on behalf of your local mechanic, if they didn't ask you to do it. It seems like a huge can of legal worms that's best not to pursue. Especially when you consider that, once you release something digitally and for free, you can't un-release it if it turns out to harm your business. Think about all the people who lost jobs because they weren't careful about what they posted, once, on Facebook--nothing really goes away.
Keep in mind, I'm coming from a software world, which has changed its mind on distribution more times than you can probably imagine. I missed the boat on "software is just a freebie that sells hardware," but I can remember working on keyed copy protection (useless once you can copy a lot of information, like a whole disc), supplemental copy protection (dongles and "type the fifth word..." junk), benign neglect, charging mainly for support, and now circling the wagons and hoping for subscriptions. For all the effort and resources poured in, nobody has made a single case that pirates are hurting the bottom line...but likewise, nobody has shown that they don't. It's all instinct and emotional, which is a bad way to enact policy change.
(Now, newspapers, I don't understand. They're still positively draconian about their intellectual property, even when it can only do a handful of people any good worth money or even time. Because, y'know, the story is out of date as soon as it's printed. So why make me jump through hoops to get at the archives when it's free to access at my local library anyway?)