<p>I'm interested in self-studying these two subjects and have narrowed down my textbook selection to "Principles of Economics" by N. Gregory Mankiw. However, I was wondering if it would be necessary to also purchase some of his other titles, particularly "Principles of Microeconomics," "Principles of Macroeconomics," "Brief Principles of Macroeconomics," and "Essentials of Economics." Would these additional titles be necessary? Also, is it suggestable that I get a book with an Xtra card? I appreciate all those who respond. Cheers.</p>