Here's another reference:
http://law.onecle.com/uscode/17/index.html
One question might be whether in downloading an epub library book you have implicitly or explicitly agreed to only use the "proper" reader application.
And in googling, I also came up with the 5th Circuit's decision.
It will take the Supremes to decide this ultimately. That is if anyone is willing to chance where they come down. Right now there is a CLUB and some folks might not want to find out it is made of foam. Keeping folks concerned about law breaking may be better than chancing they might not be in the final hour.