The former bishop of Oxford Richard Harries nominates The Cloud of Unknowing, an anonymous 14th century text, among others:
According to Christian theology there are two ways to know God. There is what’s called the positive way, where you try to say things about God, and then there is the negative way, where you have to unsay everything you have said about God and go into a kind of unknown. This is because everything we say about God can be somewhat misleading because we talk in metaphors all the time. And metaphors are always as untrue as they are true. They have to be broken and then remade and broken and remade. And therefore to know God as he truly is, instead of purely our projection, we need to go beyond all our metaphors into what he referred to as “the cloud of unknowing”, where you simply reach out to God himself, whom you can’t characterise or describe at all.
My thoughts about the best writers on God here.
The Dish 