Merge pull request #1810 from impr0t/master

Added two new titles to the Mathematics category.

free-programming-books.md

@@ -393,7 +393,9 @@ Original Source: [List of freely available programming books](http://web.archive
 #### Mathematics
 
 * [A First Course in Linear Algebra](http://linear.ups.edu) - Robert A. Beezer
+* [Advanced Algebra](http://www.math.stonybrook.edu/~aknapp/download/a2-alg-inside.pdf) - Anthony W. Knapp (PDF)
 * [An Introduction to Statistics](http://work.thaslwanter.at/Stats/html/) - Thomas Haslwanter (using Python)
+* [Basic Algebra](http://www.math.stonybrook.edu/~aknapp/download/b2-alg-inside.pdf) - Anthony W. Knapp (PDF)
 * [Bayesian Methods for Hackers](https://github.com/CamDavidsonPilon/Probabilistic-Programming-and-Bayesian-Methods-for-Hackers) - Cameron Davidson-Pilon
 * [Calculus Made Easy](http://www.gutenberg.org/ebooks/33283) - Silvanus P. Thompson (PDF)
 * [CK-12 Probability and Statistics - Advanced](http://www.ck12.org/book/Probability-and-Statistics---Advanced-%2528Second-Edition%2529/)