"Why Christianity Must Change or Die" was easily the most radical book that I have read recently. In this book, Spong (an Episcopal Bishop) essentially takes a sledge hammer to every sacred cow of the historical church. For example, the first chapter basically argues that the Apostle's Creed is barbaric and blatantly masculine, and the church would do well to leave it where it belongs in the 4th century. For orthodox Christians, reading this book leaves you consistently questioning whether or not Spong is intentionally being a heretic, which appears to be a role that he is comfortable embracing.
I actually strongly agreed with Spong's thesis that the Church must be radically transformed or it is in danger of becoming completely irrelevant. I also appreciated that he addressed his book to "believers in exile," because there are a good number of people, including myself, who resonate with this type of identity. My central problem with the book was the Spong spent a lot of time being destructive and not enough time being constructive. His attempts to describe the direction the Church should be going and the language it should be using were vague and not very plausible. To his credit, he clearly admitted that he did not really have answers and that he was merely trying to name an existing problem. I am not sure if Spong has written much since this book was published, but finishing the book certainly left me wishing he had more to say about where the Church needs to go.