Although by no means exhaustive, here's a short list of recent articles that offer good critiques of Plantinga's externalist analysis of epistemic warrant:
Dawson, Shawn. "Proper Functionalism: A Better Alternative?" Religious Studies 34 (1998), pp. 119-34.
Botham, Thad. "Plantinga and Favorable Mini-Environments", Synthese 135:3 (2003), pp. 431-41. Here Botham shows that Plantinga's amendments to his analysis of warrant since the publication of his Warrant and Proper Function (e.g., his clause requiring the beliefs to be formed in favorable cognitive mini-environments) are subject to new counter-examples.
Bardon, Adrian. "Reliablism, Proper Function, and Serindipitous Malfunction", Philosophical Investigations 30:1 (2006), pp. 45-64.
-"Two Problems for the Proper Functionalist Analysis of Epistemic Warrant", Southwest Philosophy Review 15 (1999), pp. 97-107.
Wunder, Tyler. "Anti-Naturalism and Proper Function", Religious Studies 44 (2008), pp. 209-224. This one's in on a technicality. It's not devoted to his analysis of warrant (although his doctoral dissertation is). However, Wunder does raise a counter-example to Plantinga's analysis of warrant as a means to making another point. Furthermore, (and again, though he doesn't use the point in quite this way) Wunder's paper brings up a fundamental problem with Plantinga's approach that's also broached in Botham's paper, viz., that Plantinga's analysis of warrant fails to meet standard desiderata of a proper philosophical analysis of a property.
While I'm at it, here are a couple of books that offer shorter critiques of Plantinga's analysis of warrant:
Feldman, Richard. Epistemology (Prentice Hall, 2002). See ch. 5, pp. 99-106. This is probably the best point of entry into critiques of Plantinga's account, as it's a primer on epistemology, and so the critique is presented much less rigorously than the other texts.
Connee, Earl and Richard Feldman. Evidentialism: Essays in Epistemology (Oxford, 2004). See, especially, their discussion under the heading, "Authoritarian Epistemology". There, they argue that his analysis is inadequate, as it suffers from a problem analogous to the Euthyphro Dilemma problem for divine command theories of ethics. See also their stuff on Plantinga's notion of "impulsional evidence".
 Though he uses the point about Plantinga's standards a little differently. One of Wunder's points is that Plantinga has two different standards of what constitutes an adequate philosophical analysis of a property -- a very loose one that he applies to his own account of warrant, and a very strict one that he applies to everyone else's account of anything -- and that he argues for his own account by illicitly applying the loose account to his own analysis, and the strict account to everyone else's.
On his loose standard of analysis, one states a set of conditions that hold, at least for the most part, to a set of clear, paradigm cases. These serve as the exemplars for the "core" notion of the analysis. However, the conditions need not all apply to every entity that falls under the proposed analysis. For the account allows for there to be, in addition to the core cases, a surrounding "penumbral belt" of cases that stand in various degrees of similarity to those in the core. And the idea is that since the cases are only analogous to the core cases, it's understandable that some of the conditions stated in the analysis do not apply to at least some of the penumbral cases. Furthermore, beyond the penumbra are a large number of borderline cases that bear an even fainter resemblance to the paradigm cases. These are cases where it's either unclear that the cases fall under the concept analyzed, or there just is no determinate fact of the matter whether they do. And of course, if this is so, then some of the clauses may well fail to apply to these as well. One can imagine that "analyses" of this sort will often be messy, with many long, inelegant clauses embedded within them.
In contrast to his loose standard of analysis is his strict standard. According to the strict account, an adequate analysis must state a short list of individually necessary and jointly sufficient conditions, and these conditions are not restricted to a set of "core" cases; they must apply in each and every possible instance of the property being analyzed (no leeway for a distinction between "core" and analogically extended "penumbral" cases).
On the strict-standards account of an adequate analysis, then, it's sufficient to defeat an analysis of a property if one can think up even a merely epistemically possible scenario in which the analysis fails. Thus, on this account, it's very, very easy to defeat a proposed analysis. This is the standard that Plantinga applies to every one else's philosophical accounts.
By contrast, on the loose-standards account, providing an epistemically possible scenario at which the analysis fails isn't nearly enough to defeat an analysis. Indeed, on this account, it's not necessarily enough to provide a metaphysically possible scenario at which the analysis fails. For the loose-standards account allows that not all the conditions that apply to the core cases apply in the penumbral cases, or in the cases that lie somewhere between the core and the penumbra. On the loose-standards account, then, it's very difficult to defeat an analysis. This is the standard that Plantinga applies to his own account of warrant.
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