Just by way of clarification, it is not quite true to say that Reformed Epistemologists believe that belief in God can be held without justification, otherwise the title to Plantinga's book Warranted Christian Belief mentioned above would be completely off base. Rather, they believe that belief in God is justified/warranted in a non-inferential way in the sense that one's justification for belief in God need not be on the basis of other propositions that one knows or believes (e.g. the premises of a cosmological or teleological argument). In other words, it does not depend upon evidence for its justification (construed as propositional evidence). Rather, it is taken to be justified in the context of experience (e.g. the experience of the conviction of the Holy Spirit, the beauty of creation, conviction for sin, etc) in the same way that belief in the external world is grounded in our experience of it, and not in some argument or inference leading to the conclusion of its reality. We don't come to the conclusion that the world around us is real by reasoning to it on the basis of propositions already taken to be known, but rather we naturally form our belief in it in the context of our experience of it. It is a properly basic belief.
A properly basic belief is a belief that one is justified in holding apart from any other beliefs one already holds conferring justification on it. It does not depend upon any evidence or other beliefs that one holds for its justification. Rather, it is a foundational belief, from which other non-basic beliefs may derive justification. It is a basic belief, and properly so. Plantinga and other Reformed Epistemologists give many examples of such beliefs, including the reality of the external world, the belief that the universe wasn't created five minutes ago with the appearance of age (and with all our false memories implanted in us), the reality of other minds, etc. These, they contend, are all examples of beliefs that we are justified in holding apart from any evidence for them. The important thing to remember is that these beliefs are not in any way taken as arbitrary, but rather are appropriately grounded in some way in experience. Reformed Epistemoligists like Plantinga and Wolterstorff take belief in God (and more fully in the basic truths of the Christian faith) to be one such properly basic belief.
Warranted Christian Belief was already mentioned above as a resource to look into, and there probably is no better one for this topic.