I've finished reading in the last few weeks "Jesus for the Non-religous" and "Reclaiming the Bible for a Non-Religious World." I greatly enjoyed both. What I realized as I was reading both was that to get the full sense of what Bishop Spong is getting at there are several of his works that would be worth spending time with.
Anyhow, thoughts on his works that you consider must reads? (or other authors that he references?)
As a side note I also just finished "Why Did Jesus, Moses, the Buddha, and Mohammed Cross the Road?: Christian Identity in a Multi-Faith World," by Brian D McLaren and I highly recommend it.