Best book or online resource for bondage play?

So, my girlfriend and I are not hugely into bondage at the moment. Early on we experimented with light bondage quite a bit, but that's been on the backburner for a long time. I'm looking for some new stuff to try, as well as a resource for how to approach bondage in a long-term, committed relationship in which neither of us is strictly the sub or the dom. That said, I find it very sexy, and we'd like to get more into it, but we find it awkward transitioning from our normal silly, playful, and pretty even-footed relationship and into whatever hat it is we need to put on for bondage play. Any tips or pointers would be appreciated.