Those working particularly on external world skepticism will be interested in these two very recent books:
D. Pritchard, Epistemological Disjunctivism (OUP, 2012). More information can be found here.
M. Willaschek (ed.), Disjunctivism: Disjunctive Accounts in Epistemology and in the Philosophy of Perception (Routledge, 2012). More information can be found here.