While it seems problematic to come up with a top N set of best sellers in 
Economics, there is an extensive body of scholarship one can draw on 
regarding the history and economics of publishing that should allow 
one to put Henry George in some sort of perspective as a best selling 
economics author. Indeed there is a scholarly association, the 
Society for the History of Authorship, readership, and publishing 
(SHARP) -- or something like that-- that delves into such issues for 
publishing broadly.  
 
One consideration is whether publishers, such as George's maintained records 
of print runs as well as book prices. Of course there are complications 
such as George's works going through multiple editions with multiple 
publishers. But looking at evidence on print runs and numbers of printings 
for some of George's major works with some of his leading publishers 
should at least provide perspective. One can then compare the magnitudes 
with what one can glean on other likely leading economics sellers. THere 
may be  some evidence out there on numbers of copies sold of 
Samuelson's textbook in various editions as well as say Mankiw. 
 
My recollection is that Keynes intended his General Theory believe it or 
not to have a wide readership and tried to get it priced by his publisher 
accordingly. It should stand to reason that most leading authors of 
economics texts would be aware of how their books are selling and there 
should be some indications in their biographical details of the magnitude 
of the income flows involved.   I don't claim there is 
an easy answer to the issue of establishing comparable best sellers to 
progress and poverty but it is an issue one should at least be able to 
address with due archival digging.   If Victorian literature scholars can 
take up this issue for say  Dickens and George Eliot, which I am almost 
sure they have, I dont' see  why historians of economics shouldn't be able 
to pursue the same for writers such as Henry George. 
 
David Mitch