Ptolemy of Lucca & Thomas Aquinas 
On the Government of Rulers [PDF ebook] 
De Regimine Principum

Ptolemy, considered a proto-Humanist by some, combined the principles of Northern Italian republicanism with Aristotelian theory in his De Regimine Principum, a book that influenced much of the political thought of the later Middle Ages, the Renaissance, and the early modern period. He was the first to attack kingship as despotism and to draw parallels between ancient Greek models of mixed constitution and the Roman Republic, biblical rule, the Church, and medieval government.
In addition to his translation of this important and radical medieval political treatise, written around 1300, James M. Blythe includes a sixty-page introduction to the work and provides over 1200 footnotes that trace Ptolemy’s sources, explain his references, and comment on the text, the translation, the context, and the significance.