Are you concerned about people obtaining an illicit copy of your book or are you concerned with getting those who would pay to pay. There's a difference there and often there's a vast difference in the two groups.
Consider Adobe Photoshop; there's a *lot* of individuals in their teens or 20s that have a pirated copy of the software and are using it to create avatars for forums or photchops to post in funny picture threads or whatnot. If Adobe were to somehow manage to make Photoshop un-piratable, these people aren't going to go out and spend four figures for a licensed copy. They'll move on to the next best thing.
However, there's a decent business case to be made for Adobe allowing this user-group to have illegit access, that by doing so the future employment base of graphic artists and web designers etc, is self-training in their tool and as a result will drive employers to select Adobe over an alternative (when 80% of your workforce already knows tool A, why pay them to attend weeks of training on tool B?)
In fact, there's quite a few vendors of software who allow their software (usually a version without "enterprise" features) to be used for home use for free. Avast and AVG for example are anti-virus companies that both do so. They do so since the IT guy is going to recommend for the company what he knows and uses, and they're focused on corporate sales and support and don't want to provide support to every redneck Bob, they'll let McAfee or Norton deal with him.
Point is that people using your material without permission likely isn't the real problem. People using your material who *would* pay for it if they couldn't illegitimately get it for free are. I'd be careful when attacking this problem because there are upsides from allowing the group that isn't going to pay for it no matter what to have access... in Adobe's case a huge user-base pushing employers to their product. In your case perhaps simply word of mouth or good will within the community you're writing for (can be especially important in some niche communities) or something else could be at risk.
Point is before implementing anything to try to restrict those that would pay but aren't, be careful to understand what impact you'll have on those who wouldn't pay... and how that will in turn impact you.
Of course... you may disagree and have a philosophical objection to anyone having access that hasn't paid in which case ignore what I've said