Not all books get pirated. In order to get an uncracked version you have to (a) care and (b) either know that software exists and how to get it or know about torrent sites and how to get them. Know your audience.
DRM is probably nearly useless for a book like "High Performance 3D Graphics - Linux Edition", because anyone using that book is going to be tech savvy enough to understand what DRM is and how to look for cracked editions.
If, however, your book is "A History of English Sheepherding", then you are likely dealing with a less tech-savvy audience and DRM might not be worth the cost because even if cracked copies get out there, your audience won't be looking for them.
DRM is probably nearly useless for a book like "High Performance 3D Graphics - Linux Edition", because anyone using that book is going to be tech savvy enough to understand what DRM is and how to look for cracked editions.
If, however, your book is "A History of English Sheepherding", then you are likely dealing with a less tech-savvy audience and DRM might not be worth the cost because even if cracked copies get out there, your audience won't be looking for them.