Update free-programming-books-subjects.md (#8225)

* Update free-programming-books-subjects.md

* Update free-programming-books-subjects.md

* Update free-programming-books-subjects.md

books/free-programming-books-subjects.md

@@ -31,6 +31,7 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 * [Mathematics For Computer Science](#mathematics-for-computer-science)
 * [Misc](#misc)
 * [Networking](#networking)
+* [Object Oriented Programming](#object-oriented-programming)
 * [Open Source Ecosystem](#open-source-ecosystem)
 * [Operating Systems](#operating-systems)
 * [Parallel Programming](#parallel-programming)
@@ -643,6 +644,11 @@ Books that cover a specific programming language can be found in the  [BY PROGRA
 * [ZeroMQ Guide](http://zguide.zeromq.org/page%3Aall)
 
 
+### Object Oriented Programming
+
+* [Object Oriented Programming](https://www.cl.cam.ac.uk/teaching/0910/OOProg/OOP.pdf) - Robert Harle (PDF)
+
+
 ### Open Source Ecosystem
 
 * [Contributing to opensource: the right way](https://github.com/Mte90/Contribute-to-opensource-the-right-way) - Daniele Scasciafratte