Find yourself a copy of '1491' by Charles C Mann
http://www.bookbrowse.com/reviews/index.cfm/book_number/1649/1491
It is a culmation of 20th Century research & summarizes/examines several important schools of thought of critical questions about the people of America before the European invasion of the 16th Century. Mann suggests the best evidence supports the idea of a population of some 40 millions in North America in the 15th Century. The proponents of that number are drawing from latter 20th Century archeological evidence of villiage size & density, burial site density, the nominal ability of the agricultural techniques of the era to feed people, and eveidence from the very earliest Spanish exploration into the central Mississippi basin, such as the ill fated Navaro expedition.