I think that personalized copies of e-books in non-obstructive but easily identifiable way would work much better than DRM. It would discipline people to not give their purchased e-books even to friends because there is a risk that they may inadvertently put them on torrents for public download and then you can be hold responsible for it.
Oh, by the way, I was wrong about the crack for that game. My wife says that she only intended to use it but it was not necessary. The company apparently had received too many complaints about their DRM problems so they had issued an update for the game to allow playing it without the CD inserted. Complaining about DRM can sometimes change the minds of vendors after all.
I believe that e-books have DRM only because the parties involved in their publishing do not fully understand the actual results of DRM use. It is only a time when most ebooks will be sold without DRM.