I recommend to you two posts on divine impassiblity from the Reformation 21 blog. I think this doctrine is widely misunderstood, even in evangelical circles. In fact, if the emergent/postmodern church types got this right, I think half of their problems would vanish.
Phil Ryken's post is here and Derek Thomas' post is here. The posts are short, but they bear reflection.
While I'm linking, let me point you to some more free audio from the OKC Conference on Reformed Theology. The 2006 speaker was Michael Horton. The archives have free MP3 downloads and great speakers. Check them out!
h/t Justin Taylor