I still haven't found the study, but I did find a reference to the study--only this just mentions the number of books that are bought and actually opened (hint: 60 percent):
http://goodereader.com/blog/e-book-n...ey-buy-anymore
I'm pretty sure that's part of the same Kobo study that looked at how many free books were ever opened--I just can't find the full study, just bits about it. I may stumble across it eventually, but for free books it was much much lower.