Merge pull request #162 from corragon/patch-4

Added two books in Mathematics section

free-programming-books.md

@@ -268,6 +268,8 @@
 * [Mathematics for Computer Science](http://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-042j-mathematics-for-computer-science-fall-2010/readings/)
 * [Category Theory for Computing Science](http://www.math.mcgill.ca/triples/Barr-Wells-ctcs.pdf) (PDF)
 * [Essentials of Metaheuristics](http://cs.gmu.edu/~sean/book/metaheuristics/) by Sean Luke
+* [Advanced Data Analysis from an Elementary Point of View](http://www.stat.cmu.edu/~cshalizi/ADAfaEPoV/)
+* [Probability and Statistics Cookbook](http://matthias.vallentin.net/probability-and-statistics-cookbook/)
 
 ####Misc
 * [97 Things Every Programmer Should Know](http://programmer.97things.oreilly.com/)