Reading about the party platforms
Here are several articles that evaluate the party platforms, compare the Democratic platform to the Republican platform, and/or analyze how one of the parties' platforms over time to explain how that party's position on major issues has changed. They will all be useful in helping us to determine whether the 5th party system is over, if there has been a 6th party system (or a 7th), how to describe the current party system, and what we might anticipate for the party system in the near future.
the New York Times on how the party platforms differ
the Washington Post, "Democratic Party Platform: Uneven Progression Over the Years"
the Washington Post, "GOP Platform Shifts from Moderate to Conservative"
the New York Times on GOP's Journey from the Liberal Days of Nixon
the Washington Post, "Democrats Restore Platform Language on Jerusalem"
I'll go back and add to this list from time to time.