Harris, Charles B. Passionate Virtuosity: The Fiction of John Barth. Urbana: U of Illinois P, 1983. 
---.   Contemporary American Novelists of the Absurd. New Haven: College & Univ. P, 1971.
---.  "John Barth and the Critics: An Overview," 3-13. Waldmeir.
Tatham, Campbell.  "John Barth and the Aesthetics of Artifice."   Waldmeir.
Majdiak, David.  "Barth and the Representation of Life" 96-110.   Waldmeir.
Morrell, David. John Barth: An Introduction. Univ. Park: The Penn State UP, 1976.
Noland, Richard W.  "John Barth and the Novel of Comic Nihilism," 14-29. Waldmeir.
Waldmeir, Joseph J. Critical Essays on John Barth. Boston: G.K. Hall & Co., 1980.
Ziegler, Heide. John Barth. New York: Methuen, 1987.
Glaser-wohrer, Evelyn. An Analysis of John Barth's Weltanschauung: His View of Life and Literature. Salzburg, Austria: Institut Fur Englische Sprache Und Literatur, 1977.

• John Barth, by Brian Green.
• Virtuality

• A summary of "The Literature of Exhaustion." Note: the first sentence of this text incorrectly states that Barth's essay "proposed that the conventional modes of literary representation had been 'used up'..." though this is a quibble; the text as a whole is a fair summary, in my opinion, of Barth's argument.
• "Study notes" and questions for Lost in the Funhouse, among others.
• This looks interesting: The John Barth Lecture Hall, where you can post "contemplations, musings, and queries" about John Barth, or visit the John Barth Live Recitation chatroom.

Postmodernism and the Postmodern Novel at The Electronic Labyrinth.