Merge pull request #846 from esparta/master

[en] Adding an operating systems book

free-programming-books.md

@@ -428,13 +428,14 @@
 
 ####Operating systems
 * [How to Make a Computer Operating System (Work in Progress)](https://github.com/SamyPesse/How-to-Make-a-Computer-Operating-System)
-* [The Art of Unix Programming](http://catb.org/esr/writings/taoup/html/) - Eric S. Raymond
+* [Operating System Concepts, 8th Edition](http://elajt.info/lit/OS_8th_Edition.pdf) (PDF)
-* [The Little Book of Semaphores](http://greenteapress.com/semaphores/) - Allen B. Downey
 * [Operating Systems and Middleware](https://gustavus.edu/mcs/max/os-book/) (PDF and LaTeX)
 * [Operating Systems: Three Easy Pieces](http://pages.cs.wisc.edu/~remzi/OSTEP/) (PDF)
 * [Practical File System Design:The Be File System](http://www.nobius.org/~dbg/practical-file-system-design.pdf) (PDF) - Dominic Giampaolo
+* [The Art of Unix Programming](http://catb.org/esr/writings/taoup/html/) - Eric S. Raymond
+* [The Little Book of Semaphores](http://greenteapress.com/semaphores/) - Allen B. Downey
+* [UNIX Application and System Programming, lecture notes](http://www.compsci.hunter.cuny.edu/~sweiss/course_materials/unix_lecture_notes.php) (PDFs) - Prof. Stewart Weiss
 * [Xv6, a simple Unix-like teaching operating system](http://pdos.csail.mit.edu/6.828/2012/xv6.html) (PDF)
-* [Operating System Concepts, 8th Edition](http://elajt.info/lit/OS_8th_Edition.pdf) (PDF)
 
 
 ####Parallel Programming