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