Update free-programming-books.md

Add "The little book about OS development"

free-programming-books.md

@@ -768,7 +768,7 @@ Original Source: [Free Programming books](http://stackoverflow.com/revisions/392
 * [The New C Standard - An Annotated Reference](http://www.knosof.co.uk/cbook/cbook.html)
 * [TONC GBA Programming - Game Boy Advance Development](http://www.coranac.com/tonc/text/toc.htm)
 * [Using C on the UNIX System - David A. Curry](http://www.bitsinthewind.com/about-dac/publications/using-c-on-the-unix-system)
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+* [The little book about OS development - Erik Helin, Adam Renberg] (http://littleosbook.github.io/)
 
 ### C Sharp
 * [Beginners Guide to C# and the.NET Micro Framework](http://www.ghielectronics.com/downloads/FEZ/Beginners%20guide%20to%20NETMF.pdf)