Excluding religious works what is the worlds top selling book ? The answer is

Spreading Knowledge Across the World

USA - United States of America  Canada  United Kingdom  Australia  New Zealand  South America  Brazil  Portugal  Netherland  South Africa  Ethiopia  Zambia  Singapore  Malaysia  India  China  UAE - Saudi Arabia  Qatar  Oman  Kuwait  Bahrain  Dubai  Israil  England  Scotland  Norway  Ireland  Denmark  France  Spain  Poland  and  many more....