Merge pull request #389 from srilman/master

Added free books and courses

extras/free-books.md

@@ -77,8 +77,11 @@ Name | Author(s)
 [Grinstead and Snell’s Introduction to Probability](https://math.dartmouth.edu/~prob/prob/prob.pdf) |Charles M. Grinstead, J. Laurie Snell
 
 ### Computer Architecture
+[The Elements of Computer Systems - From NAND to Tetris](http://www.nand2tetris.org/course.php) |Noam Nisan, Shimon Schocken
 
 ### Operating Systems
+:-- | :--: 
+[Operating Systems: Three Easy Pieces](http://pages.cs.wisc.edu/~remzi/OSTEP/) |Remzi H. Arpaci-Dusseau, Andrea C. Arpaci-Dusseau
 
 ### Computer Networks
 

extras/free-courses.md

@@ -125,7 +125,7 @@ Courses | Duration | Effort
 Courses | Duration | Effort
 :-- | :--: | :--:
 [Certified Information Systems Auditor](https://www.cybrary.it/course/cisa/#)| - |  8.5 hours
-[The Hardware/Software Interface](https://www.coursera.org/course/hwswinterface)| 8 weeks | 10-15 hours/week
+[The Hardware/Software Interface](https://courses.cs.washington.edu/courses/cse351/16sp/)| 8 weeks | 10-15 hours/week
 
 ### Operating Systems
 
@@ -194,6 +194,11 @@ Courses | Duration | Effort
 
 ### Compilers
 
+Courses | Duration | Effort
+:-- | :--: | :--:
+[Compilers](https://lagunita.stanford.edu/courses/Engineering/Compilers/Fall2014/about)| 9 weeks | 6-8 hours/week
+[Computer Language Engineering](https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-035-computer-language-engineering-sma-5502-fall-2005/)| 15 weeks | -
+
 ### Parallel Computing
 
 ### UX Design