Bible

Great job! The corrections I have are because there are some unique cases in these sentences. "Mostly" means for the most part, what you want to say here is "most bought" but since you are using a superlative (you are talking about the highest degree of most) then we say "bought the most". Also, "the people" means specific people but "people" is more general. 

#1 - Do you know which book has been bought the most by the people all over the world?

#2 - The Bible is the book which has been bought the most by the people all over the world.