As far as I'm aware, I've never known any of my friends to be in an abusive relationship. I hope that means they've been lucky enough to not experience one, but this sort of thing still hits me.
I just wanted to share this, by Jim Hines.
Why Doesn't She Just Leave? I find Hines' posts on the subject informative and cognizant about his own shortcomings in terms of understanding. He doesn't claim to have all the answers, but shares his own experiences, his misguided assumptions, and how those assumptions were overturned.