I think it's quite uncommon to have a commercial product that is widely pirated but not widely sold. It's always going to be roughly the same proportions of sales-to-pirates for your product. if you had thousands of copies downloaded but only a handful of copies sold, it probably means it wasn't a very popular book, and wouldn't have sold well regardless of piracy or not. If pirating didn't exist, your sales numbers might not change much because the benefits of no piracy (payment being the only option) somewhat cancels out the benefits of piracy (widespread knowledge of your product).
I'm not condoning piracy, but the effects of it are complex. I'd rather replace it with a friendlier approach, like Bandcamp's pay-what-you-want options.
I don't have any citations. It's not a hard rule and doesn't apply everywhere, but I feel like it would be somewhat applicable in this situation. It's just a thing that I've concluded as a result of my own experiences with selling digital media (games and music). Piracy did indeed spread awareness of my music and game, although it's hard to say if caused any lost revenue.
I supposed some fraction of pirates would have instead paid for the media had they not been able to pirate it, but there would also be a fraction of paying customers who would not have heard about the media had their peers not pirated it. It's hard to tell if these two populations are roughly equal in size, but I have a hunch that the second group is larger. This is based on our sales data following our involvement in the Humble Bundle: http://hitboxteam.com/dustforce-sales-figures
If you look at the third chart, you can see that after the Humble Bundle spikes, the blue line starts to have an upward slope, whereas it was almost flat before. This indicates that sales on Steam increased after the Bundle finished, due to the huge influx of new players (many who only paid a penny). It seems that the more people playing the game, the more copies you will sell, because more awareness is raised. There are a lot of factors involved in that that don't let me conclusively say that this effect extends to pirates too, but it seems reasonable. Since I can't do anything about piracy, I don't let it bother me too much and just focus on making things worth buying. It's just a part of the business, so it's factored into it all.
I'm not condoning piracy, but the effects of it are complex. I'd rather replace it with a friendlier approach, like Bandcamp's pay-what-you-want options.