Add Think OS book (greenteapress)

free-programming-books.md

@@ -439,6 +439,7 @@
 * [Practical File System Design:The Be File System](http://www.nobius.org/~dbg/practical-file-system-design.pdf) (PDF) - Dominic Giampaolo
 * [The Art of Unix Programming](http://catb.org/esr/writings/taoup/html/) - Eric S. Raymond
 * [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) (PDF) - Allen B. Downey
 * [UNIX Application and System Programming, lecture notes](http://www.compsci.hunter.cuny.edu/~sweiss/course_materials/unix_lecture_notes.php) (PDFs) - Prof. Stewart Weiss
 * [Xv6, a simple Unix-like teaching operating system](http://pdos.csail.mit.edu/6.828/2012/xv6.html) (PDF)