sort_libraries.rb

class Book
  include AttributesSort
  ...
end

def sort_libraries(libraries, key)
  libraries.map do |library|
    library.attr_sort(:sort_by => key)
  end
  libraries
end
  
  
bobs_books = initialized here...
marys_books = initialized here...
toms_books = initialized here...  
  
sorted_libraries = sort_libraries([bobs_books,marys_books,toms_books],[:author,:date])