European Medieval Democracy
Suggested by Elliot Bulmer:
""Italian City Republics" by David Waley (now 3rd edition) is a very good general introduction to medieval democracy in Italy.
"Principles of Representative Government" by Bernard Manin has a good chapter on Florence.
"The Political Uses... of Sortition" by Oliver Dowlen has some good material on Florence too.
"The End of Kings: A History of Republics and Republicans" by William Everdell describes how the Florentine republic worked.
I'd also recommend Guicciardini's "Dialogue on the Government of Florence".
J. P. McCormick has written several good articles looking at the specific mechanisms for non-elite representation in Florence.
Outside of Italy, Pirenne's "Democracy in the Low Countries" was written in 1915 but still the best work on how early forms of communal / guild-based democracy developed in Flanders and Liege.