diff --git a/books/free-programming-books-subjects.md b/books/free-programming-books-subjects.md
index d525a45cd..0a3e7e742 100644
--- a/books/free-programming-books-subjects.md
+++ b/books/free-programming-books-subjects.md
@@ -740,7 +740,7 @@ Books that cover a specific programming language can be found in the [BY PROGRAM
 * [Project Oberon: The Design of an Operating System, a Compiler, and a Computer](https://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 (HTML)
 * [The Little Book About OS Development](https://littleosbook.github.io) - Erik Helin, Adam Renberg - (PDF, HTML)
-* [The Little Book of Semaphores](https://greenteapress.com/semaphores/) - Allen B. Downey (PDF)
+* [The Little Book of Semaphores](https://greenteapress.com/semaphores/) - Allen B. Downey (PDF) (CC BY-NC-SA 4.0)
 * [Think OS: A Brief Introduction to Operating Systems](https://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) - Stewart Weiss (PDF)
 * [Writing a Simple Operating System from Scratch](https://www.cs.bham.ac.uk/~exr/lectures/opsys/10_11/lectures/os-dev.pdf) - Nick Blundell (PDF)