Rob Richie and Steven Hill, "The Case for Proportional Representation."
Michael Lind, "Alice Doesn't Vote Here Anymore."
Robert Barta, "An Argument for PR from the Right."
Steven Hill, "An Argument for PR from the Left."
Arend Lijphart, "PR vs. Single-Member Districts in States."
John Stuart Mill, Considerations on Representative Government (excerpt).
Many of the most useful articles and books making the case for PR in the U.S. cannot be found on the Web. For citations on these materials, go to the Bibliography on the Case for PR.