UTF-8 and Windows-31J (Encoding::CompatibilityError)が直せない

下記のファイルで蔵書のデータの登録を日本語でして蔵書のデータを表示をすると UTF-8 and Windows-31J (Encoding::CompatibilityError)が出てしまいます。コマンドプロンプトのフォントはMS ゴシックにしてあります。
print "書籍名: "
book_info.title = gets.chomp
print "著者名: "
book_info.author = gets.chomp
を
print "書籍名: "
book_info.title = gets.chomp.encode("UTF-8")
print "著者名: "
book_info.author = gets.chomp.encode("UTF-8")
にすれば動くのですが教本の通り(下記のファイル)に書けない理由を知りたいと思い質問しました。

# -*- coding: utf-8 -*-
require 'date'
class BookInfo
  def initialize( title, author, page, publish_date )
   @title = title
   @author = author
   @page = page
   @publish_date = publish_date
  end
  attr_accessor :title, :author, :page, :publish_date
  def to_csv( key )
    "#{key},#{@title},#{@author},#{@page},#{@publish_date}\n"
  end
  def to_s
    "#{@title},#{@author}, #{@page}, #{@publish_date}"
  end
  def toFormattedString( sep = "\n" )
    "書籍名: #{@title}#{sep}著者名: #{author}#{sep}ページ数: #{@page}ページ#{sep}発刊日: #{@publish_date}#{sep}"
  end
end

class BookInfoManager
  def initialize( filename )
    @csv_fname = filename
    @book_infos = {}
  end
  def setUp
    open(@csv_fname, "r:UTF-8") {|file|
      file.each {|line|
        key, title, author, page, pdate = line.chomp.split(',')
        @book_infos[ key ] =
          BookInfo.new(title, author , page.to_i, Date.strptime(pdate))
      }
    }
  end
  def addBookInfo
    book_info = BookInfo.new( "", "", 0, Date.new )
    print "\n"
    print "キー: "
    key = gets.chomp
    print "書籍名: "
    book_info.title = gets.chomp
    print "著者名: "
    book_info.author = gets.chomp
    print "ページ数: "
    book_info.page = gets.chomp.to_i
    print "発刊年: "
    year = gets.chomp.to_i
    print "発刊月: "
    month = gets.chomp.to_i
    print "発刊日: "
    day = gets.chomp.to_i
    book_info.publish_date = Date.new( year, month, day )
    @book_infos[key] = book_info
    puts book_info.publish_date
    puts @book_infos[key]
  end
  require 'pp'
  def listAllBookInfos
    puts "\n---------------------------"
    @book_infos.each { |key, info|
      print info.toFormattedString
    puts "\n---------------------------"
    }
  end
  def saveAllBookInfos
    open(@csv_fname, "w:UTF-8") {|file|
      @book_infos.each { |key, info|
        file.print( info.to_csv( key ))
      }
      puts "\nファイルへ保存しました"
    }
  end
  def run
    while true
        print "
1. 蔵書データの登録
2. 蔵書データの表示
8. 蔵書データをファイルへ保存
9. 終了
番号を選んでください(1,2,8,9) :"
        num = gets.chomp
        case
        when '1' == num
          addBookInfo
        when '2' == num
          listAllBookInfos
        when '8' == num
          saveAllBookInfos
        when '9' == num
          break;
        else
          puts line.encoding
        end
    end
  end
end

book_info_manager = BookInfoManager.new("book_info.csv")
book_info_manager.setUp
book_info_manager.run