I'm tempted to include Karl Marx's immense Capital in this section - not as an insult, but because I have a gut reaction against much in the culture of Marxism, sparked by its echoes of evangelical Christianity. Something about the centrality of one text and one man, placed at the crossing-point of history, and the tendency of his followers to divide us all into sheep and goats according to our faithfulness to the creed. But I read the first volume of Capital (on a three-day train journey from Almaty to Moscow) and I get why people place such importance on it.
John Milbank offers a rather more sophisticated account of the unacknowledged theological roots of modern social and political theory, in his Theology and Social Theory: Beyond Secular Reason. It's a difficult read - I've not got through it - but what I've read and the conversations I've had with people more immersed in it convince me that Milbank and his associates are doing important thinking.
Radical Orthodoxy, the collection Milbank edited with Catherine Pickstock and Graham Ward, gives a sense of the intense intellectual community from which their work has come. There's a lot going on here, and I've only touched the edges of it, but in their high theoretical language, the Radical Orthodoxy group seem to be making arguments which have much in common with parts of Illich's thinking - and, by Gustavo Esteva's account, Illich was aware of and positive about their work.
Two other points which are worth knowing: Phillip Blond, the author of Red Tory and founder of thinktank ResPublica, is a protégé of Milbank, so this theological project is closer to the centres of power than it might appear. On the other hand, Rowan Williams was once Milbank's tutor at seminary, and is on the record as speculating that Radical Orthodoxy may be "good theology in the wrong hands." (The comment is from Jeff Sharlet's article, 'God's Own Knowledge', which is a useful introduction to the RO phenomenon.)
Speaking of Rowan Williams, it was easier to recommend his books before they came with a picture of him in a mitre, but Lost Icons is a work of intellectual and spiritual depth and power, a series of reflections on the theme of "cultural bereavement" at the end of the 20th century.
(As for my own metaphysical standing, I keep on close terms with thoughtful believers of several hues, while tending to assume that animism is the default human attitude to reality and anything else will most likely prove a temporary aberration. In the mean time, on the margins, I see the emergence of new vocational communities which may yet restore the social functions of religion, while operating with little of its vocabulary.)