Add OSTEP to Operating Systems section

Author: Anshul-Patidar

books/free-programming-books-subjects.md

@@ -800,6 +800,8 @@ Books that cover a specific programming language can be found in the [BY PROGRAM
 * [UNIX Application and System Programming, lecture notes](http://www.compsci.hunter.cuny.edu/~sweiss/course_materials/unix_lecture_notes.php) - Stewart Weiss (PDF) (CC BY-SA)
 * [Writing a Simple Operating System from Scratch](https://www.cs.bham.ac.uk/~exr/lectures/opsys/10_11/lectures/os-dev.pdf) - Nick Blundell (PDF)
 * [Xv6, a simple Unix-like teaching operating system](https://pdos.csail.mit.edu/6.828/2022/xv6.html) - Russ Cox, Frans Kaashoek and Robert Morris (PDF, HTML)
+* [Operating Systems: Three Easy Pieces](https://pages.cs.wisc.edu/~remzi/OSTEP/)
+
 
 
 ### Parallel Programming