Use String Interpolation

Do

public string GetTheBestBookName(Book book) { 
  return $"The Best book's Name is {book.Name}. and the author name is {book.Author}"; 
}

Don't

public string GetTheBestBookName(Book book) { 
  return "The Best book's Name is " + book.Name + " and the author name is " + book.Author; 
}