You can find interesting informations about the pre-1850 economic best sellers in the following article: Carpenter, K. (1975) "The economic bestsellers before 1850", Bulletin of the Kress Library of Business and Economics, no. 11. Your last question is raising a difficult issue since there is no such thing as an easily accepted and/or pertinent quantative criteria to rank economic texts as "best sellers". In the aforementioned article, Kenneth Carpenter defined best sellers as economic texts with more than 5 editions (if I remember it well). Although, it is an interesting try it is not wholly satisfactory: for example, we have very few reliable informations about the size of issues of economic books before 1850 (and for a lot of books after 1850 as well). Moreover, as the editing business became more industrialized (and issues bigger) overtime it is difficult to compare editions of different epochs. Another procedure would be to count citations but the practice of citing has greatly changed between say Adam's Smith time and ours, so again we are left with no satisfactory universal criteria. Last but no the least, how to understand economic best sellers? For example, is Joseph Stiglitz's Globalization and its Discontent to be included in the list (or Harriet Martineau's works to give another example) or shall we consider only "serious theoretical works"? Hence, in my point of view, your N is an unknown. Perhaps you may just say that Henry George's Progress and Poverty was one of the economic best sellers of its time and leave it with that? Best, Loic Charles