I wonder if there were any books written in the Middle Ages that argued the case for a republic versus a monarchy. On the opposite side we can find Dante's De Monarchia (and probably many others) who argued for monarchy.
Marsilius of Padua might be an example of what I am looking for - or might not. After all his work was heavily used in imperialist and royalist ideology, so he must have been a queer sort of republican, if at all. (I haven't yet read his works myself, so I hope I'm not distorting too much). Also, he seems to have been more concerned with the opposition between secular and ecclesiastical power rather than with various forms of secular government.
Let's say for time limits, anything preceding Machiavelli.