This (via PV) outlines how print backlist sales are plummeting thanks to e-books. This does not bode well for traditional publishers, since the backlist is the industry's old reliable cash cow. You don't have to spend money marketing the backlist, and if the book is old enough, you don't even have to buy the rights, because it's in the public domain. If it's used by schools, then you have a captive audience that literally has to buy it.
But what if they don't? What if they can go to Project Guttenberg and get it for free? This, I think, is going to be the main challenge posed to publishers by e-books--not the pathetic and obvious lie that e-books cost a lot to make, but the fact that e-books are making it so that publishers can no longer monetize the public domain.
Of course, they can always rip off authors, so there's hope for Penguin yet.