It isn't. It's been a while that I've seen a (English at least) book I can't find a pirated version somewhere, unless it has a really small, niche audience or i

It isn't. It's been a while that I've seen a (English at least) book I can't find a pirated version somewhere, unless it has a really small, niche audience or it just isn't published in a way that's much harder to use (vendor provided applications, etc). All it takes is one user with the know-how to remove the DRM and a willingness to share the file, which as the history of the internet shows, is almost an inevitability with even semi-popular books.