I mean, I know that with e-readers book piracy is now potentially practical but is it really a concern for publisher? I'm really curious to know how mainstream it is.

I wouldn't be surprised if the main cause of piracy for ebooks was not the price but rather that the book is not available on a particular platform or only in paperback. Besides the demographics for book readers owning an e-reader is probably older and more wealthy than the average movie pirate.

The only numbers we have are those given by amazon, but we don't know exactly which books they used to make their statistics. We also don't know what the people who didn't buy the 15$ ebooks ended up doing, maybe they didn't buy anything at all, maybe they just bought the paperback...