Best books or other reading material that focuses on the non-physical aspect of D/s?

I've got a few ebooks on BDSM that I've flipped through (The New Topping Book, The Loving Dominant, The Ultimate Guide to Kink) and while they have been interesting and informative they tend to focus on the technical skills involved such as rope tying, flogging, etc. My girlfriend and I are much more into simply the power exchange aspect. We do enjoy nice rough sex and I like to tie her up and spank her but she doesn't really have a pain fetish so we don't get too into technical impact play. So, I've been looking for some reading that focuses on building and cultivating the mental aspect of D/s. Any good suggestions?

Thanks!