The most popular philosophy book ever written was "Zen and the Art of Motorcycle Maintenance" by Robert Pirsig. Written in 1974, this bookhas sold more than five million copies. Centered around a motorcycle road trip, the book deals with philosophical themes in a narrative format.
Of the classical philosophers, Plato is among the most widely read, with "The Republic" being his most popular work, particularly since the mid-19th century.
The book is presented as a dialogue featuring Socrates and examines justice and its relation to happiness. Specifically, Plato poses the question of whether the just or the unjust person is happiest.