Added new course and fixed some info (#5868)

* rearrange korean course in alphabetical order

* change section according to the markdown syntax

* Add new section and book: operation system

* HTML syntax back to its original state.

* changed to direct link

books/free-programming-books-ko.md

 ### Index
 
-* [수학](#%EC%88%98%ED%95%99)
 * [Amazon Web Service](#amazon-web-service)
 * [Assembly Language](#assembly-language)
 * [C](#c)
@@ -14,6 +13,8 @@
   * [Node.js](#nodejs)
 * [LaTeX](#latex)
 * [Linux](#linux)
+* [Mathematics](#mathematics)
+* [Operation System](#operation-system)
 * [Perl](#perl)
 * [PHP](#php)
   * [Laravel](#laravel)
@@ -28,11 +29,6 @@
 * [Swift](#swift)
 
 
-### 수학
-
-* [기초정수론: 계산과 법연산, 그리고 비밀통신을 강조한](https://wstein.org/ent/) - William Stein
-
-
 ### Amazon Web Service
 
 * [아마존 웹 서비스를 다루는 기술](http://www.pyrasis.com/private/2014/09/30/publish-the-art-of-amazon-web-services-book)
@@ -106,6 +102,16 @@
 * [GNOME 배우기](https://sites.google.com/site/gnomekr/home/learning_gnome)
 
 
+### Mathematics
+
+* [기초정수론: 계산과 법연산, 그리고 비밀통신을 강조한](https://wstein.org/ent/ent_ko.pdf) - William Stein (PDF)
+
+
+### Operation System
+
+* [운영체제: 아주 쉬운 세 가지 이야기](https://github.com/remzi-arpacidusseau/ostep-translations/tree/master/korean) - Remzi Arpacidusseau (PDF)
+
+
 ### Perl
 
 * [2시간 반만에 펄 익히기](http://qntm.org/files/perl/perl_kr.html)