Adding some books (#2258)

* Update free-programming-books.md

* Alphabetical order

* Adding books in Mathematics and Operating Systems

free-programming-books.md

@@ -397,9 +397,11 @@
 
 * [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 the Theory of Numbers](http://www.trillia.com/moser-number.html) - Leo Moser (PDF)
 * [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
 * [Book of Proof](http://www.people.vcu.edu/~rhammack/BookOfProof/) - Richard Hammack (PDF)
+* [Calculus](http://ocw.mit.edu/ans7870/resources/Strang/Edited/Calculus/Calculus.pdf) - Gilbert Strang (PDF)
 * [Calculus Made Easy](http://www.gutenberg.org/ebooks/33283) - Silvanus P. Thompson (PDF)
 * [Category Theory for the Sciences](http://category-theory.mitpress.mit.edu)
 * [CK-12 Probability and Statistics - Advanced](http://www.ck12.org/book/Probability-and-Statistics---Advanced-%2528Second-Edition%2529/)
@@ -547,16 +549,19 @@
 
 * [Computer Science from the Bottom Up](http://www.bottomupcs.com)
 * [How to Make a Computer Operating System (Work in Progress)](https://github.com/SamyPesse/How-to-Make-a-Computer-Operating-System)
+* [How to write a simple operating system in assembly language](http://mikeos.sourceforge.net/write-your-own-os.html) - Mike Saunders (HTML)
 * [Operating Systems and Middleware](https://gustavus.edu/mcs/max/os-book/) (PDF and LaTeX)
 * [Operating Systems Concept 9th edition](http://codex.cs.yale.edu/avi/os-book/OS9/slide-dir/) - Avi Silberschatz, Peter Baer, Galvin Greg Gagne (PDF)
 * [Operating Systems: Three Easy Pieces](http://pages.cs.wisc.edu/~remzi/OSTEP/) (PDF)
 * [Practical File System Design: The Be File System](http://www.nobius.org/~dbg/practical-file-system-design.pdf) - Dominic Giampaolo (PDF)
+* [Project Oberon: The Design of an Operating System, a Compiler, and a Computer](http://people.inf.ethz.ch/wirth/ProjectOberon/index.html) - Niklaus Wirth & Jürg Gutknecht (PDF)
 * [The Art of Unix Programming](http://catb.org/esr/writings/taoup/html/) - Eric S. Raymond
 * [The Design and Implementation of the Anykernel and Rump Kernels](http://www.fixup.fi/misc/rumpkernel-book/) - Antti Kantee
 * [The little book about OS development](https://littleosbook.github.io) - Erik Helin, Adam Renberg
 * [The Little Book of Semaphores](http://greenteapress.com/semaphores/) - Allen B. Downey
 * [Think OS: A Brief Introduction to Operating Systems](http://www.greenteapress.com/thinkos/index.html) - Allen B. Downey (PDF)
 * [UNIX Application and System Programming, lecture notes](http://www.compsci.hunter.cuny.edu/~sweiss/course_materials/unix_lecture_notes.php) - Prof. Stewart Weiss (PDF)
+* [Writing a Simple Operating System from Scratch](http://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/6.828/2012/xv6.html)
 
 
@@ -1440,6 +1445,7 @@ For resources on Angular, Backbone, D3, Dojo, Ember, Express, jQuery, Knockout,
 * [The Python GTK+ 3 Tutorial](http://python-gtk-3-tutorial.readthedocs.org/en/latest/)
 * [Ubuntu Pocket Guide and Reference](http://www.ubuntupocketguide.com/index_main.html)
 * [Ubuntu Server Guide](https://help.ubuntu.com/16.04/serverguide/serverguide.pdf) (PDF)
+* [Understanding the Linux Virtual Memory Manager](https://www.kernel.org/doc/gorman/) - Mel Gorman (HTML, PDF)
 * [Upstart Intro, Cookbook and Best Practises](http://upstart.ubuntu.com/cookbook/)
 * [What Every Programmer Should Know About Memory](http://www.akkadia.org/drepper/cpumemory.pdf) (PDF)