James Harrington was an English political theorist of classical republicanism who developed a theory of government based on law rather than the power of individuals. His major work, The Commonwealth of Oceana (1656), advocated a return to a constitutional republican form of government, similar to that of ancient Greece and Rome. Read more at New World Encyclopedia.
Free full-text works of James Harrington online are available here.
Harrington’s magnum opus, Commonwealth of Oceana, is an exposition on an ideal constitution, designed to allow for the existence of a utopian republic.
Utopia is an ideal commonwealth whose inhabitants exist under seemingly perfect conditions. Find out more at Encyclopædia Britannica.
You may find the following article interesting: Imaginary commonwealths: More’s Utopia and Harrington’s Oceana.
Title page of The Commonwealth of Oceana