This article on the Book Designer is sensible and useful advice. I make my living as an author but always release DRM-free, since DRM has caused me so many problems as a consumer (in films, books, and games). It only ever penalises legitimate customers. The best thing is to treat your customers well, and not worry about the ones who wouldn't pay anyway. Who knows, they might give you a good review instead - that's worth more than the cost of a book.