Books are banned because they are seen as "unacceptable" due to the content of the book. Some banned books have been banned for including socially unacceptable behavior, such as swearing, sex,...
Do you mean from what places are they banned, e.g. schools, particular countries; or do you mean which particular institutions ban books? If it is the former, an example is Salaman Rushdie, whose...
Books are banned because of questionable content. Perhaps the book has sexually explicit material not suited for a particular age group. Or, maybe a book is banned because it contains racist material...