We've noted in the past the various stories of individual authors like Paulo Coelho and David Pogue, who showed that free (non-DRM'd) versions of their ebooks helped increase physical book sales. Then, in February, we wrote about some actual research that showed that when unauthorized ebooks get out into the wild, there is a "significant jump in sales" of the physical book. And, now there's even more evidence to support this. A recent paper by a PhD. candidate noticed that free ebooks tend to increase sales of physical books. In this case, rather than looking at "unauthorized" ebooks, it looks like they focused on authorized free ebook versions. Not all of the books increased in sales, but you can see that there are lots of variables that impact this (how are the ebooks offered, how are the physical books offered, what types of books are they, etc.). Either way, though, it seems like there's increasing evidence that when done right free ebooks can certainly increase the sales of physical books -- despite claims from companies like Attributor that unauthorized ebooks are costing publishers money.