I've just finished reading one of Rowan Williams' earlier books - The Wound of Knowledge. It's basically a review of "Christian Spirituality" from the New Testament as far as Luther and St John of the Cross.
What Williams means by "Christian Spirituality" is the experience of being a Christian, especially in terms of what it means to take up the cross and follow Jesus in terms of our minds - what it means to die to ourselves, to know nothing except Christ, what it means to be people who follow, worship and dwell in a God who became human and died. In doing so, he covers quite a lot of historical theology, including the Gnostics (and anti-gnostics), Augustine and Aquinas. I found that helpful in terms of seeing how, for example, Arianism and Gnosticism both stemmed from (Neo-)Platonism.
His style is very different to the classic "evangelical" style - he seems to try to present each view as if he agrees with it, and doesn't do much in terms of critical evaluation, rather seeking to explore tensions between views. That sometimes makes it difficult to see what he actually thinks himself - perhaps he accepts that all the writers he considers are straining after the same reality, trying to verbalise it in different ways. That means that his take on Luther, for example, is very different to the one I'm used to - his main focus is on the experience of Luther's understanding of the unworthiness of people in God's sight and God's transcendence rather than on the reformation solas
For what it's worth, I think his actual views are somewhere between those of Luther and John of the Cross (whose views I'm aiming to write about soon). His treatment of the Biblical stuff is pretty good...