Complex sentence: Machan's work usually focuses on ethics and political philosophy, specifically natural rights theory, in works such as "Individuals and Their Rights" (Open Court, 1989) and "Libertarianism Defended" (Ashgate, 2006).
Split into simpler sentences: Machan's work usually focuses on ethics and political philosophy, specifically natural rights theory. See his "Individuals and Their Rights" (Open Court, 1989) and also a more recent work, "Libertarianism Defended" (Ashgate, 2006).