These are all fine books, but I’m not sure I agree with you on the definition of liberal. As I see it, liberal Christianity resulted from the application of critical thought to Scriptural scholarship and theology. There has always been a range of responses to that.
On one end we have folks who think that we can’t ever really know what Jesus taught, and most of theology is speculation. They tend to create Christianity based on love, but without much revealed content. On the other end, we have people who use critical methods to adjust theology based on what we know of Jesus’ first century context, but who maintain the basic outlines of traditional theology.
You see this today by comparing people like Marcus Borg, who represents the first approach, with N T Wright, who represents the second. But from my point of view they are both part of the liberal movement (even though in the UK context, Wright is considered conservative), and the range between them has been present throughout its history.
It’s worth noting that even the more radical branch normally maintain both the Trinity and the Incarnation.
