I've spent this afternoon rubbing my face in the Parable of the Word in Mark 4. And then I read this by Dan Phillips: Doomed Evangelism. Makes very helpful, if sobering reading. The Parable-Principle in action as Moses meets Pharaoh in Exodus 10.
When Paul writes about this doctrine in Romans 9, with his own people group in view, he writes with devastation and tear-stained face. We struggle with this doctrine because we don't like the idea of people not believing when we preach. But the weight of Biblical evidence seems to be that God is active both in hardening as well as in showing mercy, in both cases for his glory.