You really should make your own choice, looking at thinkers of the Enlightenment such as Baron de Montesquieu, Voltaire, and Jean-Jacques Rousseau. But for the sake of an answer here, I'll focus on John Locke.
John Locke's Second Treatise on Civil Government is referred to often in modern political studies. In the Second Treatise, he laid out his plan for representational government of the sort the American founding fathers sought to set up, based on his advice. Locke's First Treatise on Civil Government doesn't get much attention anymore, but it was also a very good book. The purpose of his First Treatise was to debunk the idea of divine right monarchy -- the notion that hereditary dynasties of kings have received their authority and position by appointment from God. That notion of government had to be taken out of the way philosophically before a new plan could be built (as was then done in the Second Treatise).
Let me also say just a word about Rousseau's theory of the sovereignty of the people -- that the "general will" of the people is always right. Rousseau is often maligned as promoting an unworkable view of government run completely by democratic referendum. But if you read his famous book, The Social Contract, you'll see his view of the "general will" is more nuanced than that. It isn't just a majority ballot sort of thing. For instance, in the 2016 presidential election in America, the "general will" that was most expressing itself was that the country wasn't greatly happy about either candidate running for the office of chief executive of the country. If Rousseau's "general will" principle had been put into action, the nation might have called for a new round of nominations to produce a candidate that could have pulled the nation together rather than divisive candidates and parties pulling the country in opposite directions.